Until PHB2 came out the fighter was unarguably the weakest of the classes, and few took the class much beyond 4th level. After PHB2, however, I tend to think that the fighter has risen a few pegs on the ladder. I would now arguably suggest that the monk be considered the weakest of the classes. With the various useful and interesting feats in PHB2 the Fighter, I believe, can hold his own until around 18th level (and that only because there are no 'Ftr 20th level' pre-req feats in the PHB2).
On the other hand, the Cleric was considered the strongest class all throughout 3e. Then the revisions came in 3.5e, and the Druid jumped up past the cleric to become the number one in power. The wizard still holds his own around third strongest, although some have suggested that during 3e (not 3.5e) he should have been considered 2nd strongest instead of 3rd.
In the middle we have classes like the Rogue, Barbarian, and Ranger - which in my opinion are the ones that class balance should be based upon. The Warlock can probably also be tossed in there, although it is a bit weaker at higher levels, becoming something of a one-shot pony, if you'll excuse the pun.
I think there was a thread a while back where voting determined how the population of ENWorld viewed the power of the classes. I'll see if I can find it . . . . If I can I will post the results in this post later.