Top Four, Neck-and-Neck
1. Fighter. The important part of being a fighter, is that you figure out what the other guy is good at, and then you do something else in that particular combat. Magic items should be selected to give you ways around the more annoying things (like wall of force), or you could go for an antimagic field build, but basically, you shoot arrows at the melee monsters, and you close in for the kill on ranged death masters - no other class can do BOTH as well as a fighter, they always have to specialize in one or the other. Fighters also do very well at mounted combat, which makes them Death Incarnate as far as I'm concerned - even a monk has problems touching that bidness.
2. Wizard. Although a properly built fighter can take a wizard apart, same as anyone, a properly prepared wizard can kill just about anything else without using up too many spell slots. Not to mention scrolls, wands, and quicken spell. Heck, a wand specialist wizard can even wear armor.
3. Bard. Hate to be the naysayer here, but at 6th level and up, the bard starts to get really scary. Leadership + Inspire Courage + skills at the supporting role makes the bard the hands-down best kicker of booty. The change to an illusionist role hasn't hurt the warfare aspect, either.
4. Cleric. To be honest, the cleric is 4th by a narrow margin, since he's about 2nd best at the above three. Some people will argue that the cleric can out-melee the fighter, but that's only with preparation... and that preparation makes him less able to deal with a fighter who switches to range. The cleric can wear armor and cast spells, which makes him seem better than the wizard... except that the spells aren't that great, and wizards have armor spells. The cleric might outdo the bard at the warfare angle, but the new Inspire Courage is damned hard to argue with.
Middle Four, all pretty good
5. Barbarian. A melee fighter with comparatively marginal missile capabilities, the barbarian is beaten out by the cleric (who can also wear better armor).
6. Rogue. They have a good niche, and they do well in it, but without someone else to help flank, the rogue's combat role is vastly diminished. And even getting flanking bonuses with a comrade can be surprisingly difficult. The halfling rogue, however, built for defensive capabilities, and granted improved invisibility, is capable of frightening damage output... until he gets caught.
7. Ranger. In his specialty, the ranger will be pretty good. Like the barbarian, however, he gets beaten out by the cleric (who can also wear better armor) and is somewhat marginal outside of his specialty. Rangers can at least do cool stuff, though.
8. Druid. Nice general abilities, but still second place to the cleric, who does what the druid does, better.
Not So Good
9. Monk. The monk's role in combat is a bit too subtle for most, but he can be an absolute master of unusual tactics such as grappling, tripping, targetting spell casters, and so on. He sucks at long ranges, however, and doesn't do too well in pure melee, either.
10. Paladin. He's pretty cool, and with some good PrCs, can be spectacular for one or two hits, but ultimately, the paladin just isn't that great.
11. Sorcerer. As written, he's a weak wizard. He gets a few more spells per day, but gets nothin' for spell selection. It's like a wizard, only with the one real strength taken away.