Imam

thats a loaded question …like i suggested u read halm’s book as a primer for intra-shia debates about imamate

http://www.amazon.com/Introduction-Shi%60i-Islam-History-Doctrines/dp/0300035314/ref=sr_1_1?ie=UTF8&s=books&qid=1235812978&sr=1-1

there are many other sources avalible too

this is a more advanced source for debates on 12er shia rijal