This should give you a start:
http://en.wikipedia.org/wiki/List_of_UFC_champions
Fighters come and go, and people have varying opinions of them.
Tim Sylvia for example, is currently the HW champion, but is widely considered to be very overrated and expected to loose it with Mirko Crocop (Just signed from Pride) eventually being the one that ends up holding on to it for a while.
Chuck Liddell is holding the LHW belt, not really anyone left to challenge him. Although Quintin Jackson has a win over him (from Pride) and was just signed by the UFC. So that fight should be coming up soon.
Anderson Silva holds the Middleweight belt, after walking through Rich Franklin to win it, in a fight which he went into as a underdog. Before that he walked through Chris Leben, who was seen by many as a up and coming contender.
At Welter weight Georges St. Pierre holds the belt, the other 2 that have held it and are still active are BJ Penn & Matt Hughes. Hughes lost it to St.Pierre after a long run, and lost it previously too Penn, but Penn left the UFC after winning it and then lost to Hughes in his comeback attempt. Diego Sanchez will likely get a title shot here soon as he has goe undefeated since he was on the first Ultimate fighter series. Some people think he is overrated, but he's still undefeated and has been fighting some good fighters.
Sean Sherk has the light weight belt, but the division was just reintroduced and he hasn't defended ityet. There is some talk of BJ Penn dropping to LW, which is his more natural weight class, in which case he will probably take the belt.
There is also Matt Serra (MW) and Travis Lutter (LHW) that are getting title shots for winning the Ultimate Fighter series, what chance they got is debateable, but I don't think its very good. Serra is a longshot, Lutter a even longer one IMO.