the most powerful class is the one whose abilities and trsaits are most often the best solutions (or at least decent solutions) to the challenges the GM presents. since the nature and specifics of these challenges will vary greatly from individual campaign to individual campaign, so will the answers.
imagine the different needs for games featuring a war against frost giants instead of an invasion by drow instead of overthrowing a lich king's undead army instead of a quest into remote wilderness regions populated by hostile barbarian/fighter/druid tribesmen and their bestial minions. in each of these a different set of needs and different set of common challenges will be presented and different solutions found.
The last DND game i ran was 3.0 and it ran for three years with levels ranging from 3-16. it had a gnome sorcerer, an elven rogue/wizard, a dwarven cleric, a human druid/ranger, a human monk, a human barbarian, a dwarven fighter, and an elven rogue. Over the length of the campaign, we did not see balance problems of any significance come up. Everyone had plenty of times they were vital to the story and success and times where they were supporting the guys who were. It varied month by month who was in the key roles as the stories did and the challenges the Gm presented did.
IMO, the DnD classes are built with enough differences that if an imbalance is seen in play it is due to the choices of cvhallenges the GM presents favoring one character over another. No DND class is so far above any other that no matter what he is superior, has the better answer, and so forth.
Now, in that campaign i did see several things come out as patterns.
The fighter, barbarian, sorcerer, rogue were very frequently driving the action. By this i mean they did enough damage or had enough impact in fight scenes (not so much other scenes) that it quickly became evident "if i don't stop/counter those characters and what they are doing, they will win this fight." These character did a lot of damage and often could be difficult to damage bac by comparison and when hurting the other guy was the goal of the day, they were often strong enough to simply be the driving force.
By contrast, the cleric and the druid and the monk were characters who were rarely driving the action, but were supportive/useful in almost any situation. They did drive the action in more rare occasions, but not terribly so.
The cleric, in sipte of its many praises here, is IMO a very versatile class, able to have something to do in most situations, but thats not the same as power. The sorcerer and the fighter and the rogue get my bets for being the most powerful, based on 3.0 experience. that said, i understand that each campaign will likely see different answers.
Did this change in 3.5, sure i bet it did, but I really did not see gross differences in the 3.5 classes come in balance wise, so i would be VERY surprised if it has now become so skewed that its not anymore a case of "determined by the challenges the Gm throws." but hey, if it has, then that just makes my choice not to buy into 3.5 even sweeter.
