Twelver (Arabic: اثنا عشرية, romanized: Athnā‘ashariyyah or Ithnā‘ashariyyah) or Imamiyyah (Arabic: إمامية) is the largest branch of Shia Islam. The term Twelver refers to its adherents' belief in twelve divinely ordained leaders, known as the Twelve Imams, and their belief that the last Imam, Muhammad al-Mahdi, lives in occultation and will reappear as the promised Mahdi.
The Prophet Muhammad (SAWA) has clearly said: My successors will be twelve. (Saheeh Bukhari, Hadeeth number 6682, Saheeh Muslim, Hadeeth number 3393, Tirmithi, Hadeeth number 2149 and hundreds of Sunni books as well as Shia books. The names of he twelve Imams were mentioned by the Prophet himself. We believe and follow the Prophet Muhammad (SAWA) by believing and following the Twelve Imams after him. The forst Imam is Ameerul Mo'mineen Ali (AS) and the last Imam is Imam Al-Mahdi (AS).
Any other sect who claim Imams apart from teh Twelve Imams need to proof their claim from authentic Hadeeths. We have not seen any authentic evidence about Imams apart from the Twelve Imams.
Ismailies, Bohras etc are Muslims like any other Muslim sect.
1. Zayd bin 'Ali ash-Shahid:
Among the children of Imam 'Ali Zaynul 'Abidin (a.s.), after Muhammad al-Baqir, Zayd was the most outstanding and the most learned. Shaykh Mufid describes him as "a devout worshipper, pious, a jurist, God-fearing and brave." (al-Irshad, p. 403) It is worth mentioning that he is also the first narrator of the famous as-Sahifah as-Sajjădiyya of Imam Zaynul 'Abidin (a.s.).
Zayd led an armed rebellion against the Marwanid (the Umayyid clan which came to power after Yazid) ruler, Hisham bin 'Abdul Malik; and was calling people towards "the accepted person from among the descendants of the Prophet". He led the uprising in Kufa but was killed on 2nd Safar in 120 A.H. at the age of forty-two by Yusuf bin 'Amr ath-Thaqafi (the Umayyid governor), his body was taken out of the grave, put on a cross for four years, then it was burnt and his ashes were spread in the wind. (See al-Mufid, al-Irshad, p. 404; al-Mas'udi, Muruj adh-Dhahab; al-Qummi, Muntahal Amăl, p. 36).
Because of his jihad and his claim for the Ahlul Bayt, some Shi'as, however, thought that Zayd was claiming imamate for himself and therefore started believing in him as the Imam.
The Ithna-'Ashari sources do not believe that Zayd claimed imamate for himself. For example, Shaykh Mufid, one of the earliest Shi'a theologians says, "However that was not his intention because he knew of the right of his brother, peace be on him, to the Imamate before him, and of his bequest of trusteeship (wasiyya) at his death to Abu 'Abd Allăh (i.e., Ja'far as-Sadiq), peace be on him." (al-Irshad, p. 404).
Even the way Imam Ja'far as-Sadiq (a.s.) reacted to Zayd's martyrdom shows the uprightness of the latter in his faith in the Imams of Ahlul Bayt. When Imam as-Sadiq was informed about Zayd's martyrdom, "he was very sad...and he set apart a thousand dinars of his own money for the families of those of (Zayd's) followers who were killed with him." (al-Irshad, p. 405) For other such narrations by Shaykh as-Saduq, see Muntahal Amăl, p. 36.
In conclusion, we can say that Zayd bin 'Ali was an outstanding Shi'a, a mujăhid and a shahid who was loyal to the line of the Imams of Ahlul Bayt, including his own brother, Muhammad al-Baqir, and, his nephew, Ja'far as-Sadiq (a.s.). This leaves us with no choice but to reject the statement made by the late 'Allamah Tabătabă'i that Zayd himself "considered the first two caliphs, Abu Bakr and Umar, as their Imams." (Shi'a Islam, p. 77)
2. The Zaydiyya Sect:
Among the three sub-sects of the Zaydiyya, al-Jărudiyya is extinct. The other two sub-sects, the Sulaymăniyya and the Batariyya, cannot be technically considered as "Shi'a". Both believe that the Prophet did not appoint anyone as his successor; both believe in the caliphate of Abu Bakr and 'Umar but not in the caliphate of 'Uthman; they do not believe in the infallibility of the Imams; they believe that it is possible to have two imams at the same time but in two different regions.
According to the Zaydiyya, any descendant of the Prophet (i.e., a sayyid) who is a jurist (faqih, mujtahid), pious, courageous, and calls people towards Allăh by the "sword" (i.e., jihad) can be the imam. (On this account, the late Ayatullah al-Khumayni was definitely fulfilling all these requirements for the imamate of the Zaydis! I wonder what the Yemeni Zaydis have to say about this? But, on the other hand, they might say that this would apply to the Zaydis in Iran only!!)
All historians of religion, Shi'ahs and Sunnis, say that the Zaydis follow the Mu'tazila school in their beliefs, and the Hanafi school in their laws. As such, the Zaydis are more closer to the Sunnis than the Shi'as. (For details, see S.S. Akhtar Rizvi, "Shi'a Sects" published in The Light, and also reprinted in The Right Path [Toronto] in 1995).
3. Zaydi States:
It is true that one of the earliest states founded by the descendants of Imam 'Ali (a.s.) was a Zaydi state, but it was not necessarily a Shi'a state for the reasons mentioned above. Năsir al-Utrush, a descendant of the brother of Zayd ash-Shahid, arose in Khurasan. After being pursued by the 'Abbasids, he fled to Mazandaran (Tabaristan) whose people had not yet accepted Islam. "
After thirteen years of missionary activity in that region he brought a large number of people into the Zaydi branch of Islam. Then in the year 301/913 with their aid he conquered the region of Mazandaran, becoming himself Imam." (Tabătabă'i, Shi'a Islam, p. 77) The Zaydi rule in Tabaristan continued until 1126 C.E.
The Idrisi dynasty (from 788-985 C.E.) in Morocco was not a Zaydi dynasty. It was founded by Idris bin 'Abdullah, a great-grandson of Imam Hasan bin 'Ali (a.s.).
4. What is our response?
Since the Zaydiyya believe in the caliphate of Abu Bakr and 'Umar, our response and arguments with them should not be any different from our arguments for the Sunnis.
It is difficult to answer the question of "who is right" since everyone thinks their religion is right! Answering the question of "who is orthodox?" also brings up the question of who has the right to determine what is orthodox.
(Keeping in mind many Muslims consider all Shi'is to be unorthodox!)
Anyway, as you say, many people today consider the strongly fiqh-based approach to Twelver Shi'ism (i.e. the first stream of thought, in that Shaykhism also emerges from that line) to be the "mainstream" or "orthodox" one.
It is also argued by scholars of religious history that Alawism and Alevism are heavily syncretic religions (i.e. strong mixes of Islam/Shiism and other religions). My understanding is that some Nimatullahis self-identify in this way today (that is, as strongly integrating beliefs prior to Islam), but I don't know if that reflects the whole tradition, and, in any case, just because a belief is ancient does not necessarily mean that it is wrong. However, it could be construed as "less orthodox" insofar as it there is no evidence that it comes down a direct line from the Prophet/Twelve Imams.
My impression has always been that today's Alawism and Alevism are more "cultural religions", i.e. they are practiced in some areas as local traditions but don't absorb outsiders easily.
Anyway, my view on this is, firstly, to follow the advice of Imam Ali (A) - namely, first know the truth, and then you will come to know the people who are on the truth.
Second, insofar as Islam is a scriptural religion, you can read Qur'an and hadith, especially the Qur'an, and compare the beliefs and practices of these different groups and see what seems to fit best with it.
Third, there is no harm in taking what is good from different places. If you have the option to practice Shi'ism in one or more of these interpretations, you can see what leads you to the truth, what beliefs and practices are healthy or unhealthy, which reflect the spirit of the Qur'an, what you think best reflects the intent of the Prophet (S), etc.
Lastly, of course, seek divine guidance.
Usually we already know what is true and the kernel of the answer is already in our heart, but sometimes we aren't ready to act on it yet, and we have to wait until the time when we are ready to acknowledge whatever we know is true.
People have always sought to arrange people into groups and to declare which are correct or not correct, or orthodox or not orthodox. First of all, what do you mean by 'orthodox'? Secondly, the Usuli and Akhbari both accuse each other of heterodoxy. Both are Twelver Shi'a. The Shaykhis are Twelver Shi'a and the Ni'matullahis are Sufis and are also Twelver Shi'a, except that one branch of the Ni'matullahis praises 'Umar ibn al-Khattab. Much Sufism that developed among the Shi'a during the medieval period incorporated elements of the Sunni Sufi silsilas, yet practitioners of that Sufism would consider themselves to be Twelver Shi'a. There are different 'Alawi groups - so which ones are you referring to? In the early period, the name 'Alawi was just another name for the Shi 'a of 'Ali [as]; the term 'Alawi also came to be use for the descendents of Abu Talib's [ra] household; there are also Sunni 'Alawis descended from Imam al-Hasan [as]. You say which ones are correct and by what criteria: correct from what perspective? A Sunni perspective? A Shi'i perspective? A secular perspective? Please clarify.
This statement is not correct.
The name of Twelvers came from the Prophetic statement that my successors will be twelve. This most authentic Hadeeth is narrated in main Sunni books including Bukhari, Muslim, Tirmithi, Ibn Maajah, Musnad Ibn Hanbal hundreds of Hadeeth books.
While all Muslim scholars believe that the Prophet said this, but only Shia Muslims follow the Prophetic teachings by following the Twelve successors.
In fact every Muslim must believe and follow the Twelve Imams If he really wants to abide to the Prophetic orders.
The Osooli and Akhbari and Shaikhi are not different sects, as all of them believe and follow the Twelve Imams from Ahlul Bayt (AS), but some of their scholars have their attitude to derive the Islamic rules from the Hadeeths.
The criteria to know the truth among all the different opinions and sects is the authentic evidence from Quran and authentic Hadeeth.
From the Shi'a point of view an Imam is not chosen by people. Even the ma'soom Imam himself does not chose who the next Imam will be. It is something exclusively appointed by Almighty God.
We clearly know that an Imam must have certain characteristics, the most important of them is 'Ismah, which is not something that can be acquired, and it is something endowed by the Almighty.
In addition to this, we see that the Prophet had mentioned who the Ahlul Bayt (a.s.) were, and the numerous famous traditions of the Twelve Khalifahs have also mentioned this.
Even in the case of Imam Husain (a.s.), it was one of his sons who was divinely selected and appointed to be the next Imam.
Another point that can be mentioned here is that the descendents of Imam Hasan (a.s.) did have a continuous role in Imamah.One example for this is that Imam Muhammad al-Baqir's (a.s.) mother was the daughter of Imam al-Mujtaba (a.s.). This means that Imam al-Baqir (a.s.) goes back to both Imam Hasan and Imam Husain (a.s.).
There is a tradition that says as a result of the greatest sacrifice made by Imam Husain (a.s.), he was blessed with three things:
1. The lineage of the Imams are from him.
2. There is cure in his soil.
3. Prayers are answered under his dome.
May the Almighty grant us their shafa'ah.
"Shia", in his technical sense, originally indicated a person who followed Ali and preferred him to the other sahabah. It was not supposed to be a sect or a variety of doctrinal branches with different theological or religious views. As a matter of fact divisions were spread, the book "Firaq al-Shia" by Hasan Ibn Musa al-Nawbakhti, available also in English, mentions most of their ramifications. Nowadays the main surviving schools are the Zaydiyyah (Zaydis), the Ismailiyyah (Ismailis) and Ithna Ashariyyah (Twelvers).
With prayers for your success.