I'll rate classes according to mechanics here, because that is what a class basically is. After all, you can adjust the flavor all you want to make your character special - but the mechanical skeleton must still be able to support your idea of your PC. I'll give comments like 'great flavor' from time to time, but these should not be taken for too much.
Complete Warrior has:
Samurai - a below average melee type whose class abilities don't really synergize well, even if the oriental flavor is OK with your campaign. I'd skip this. If you need to know: he gets to fight with two weapons (daisho style), and can impress and frighten opponents.
Hexblade - a fighter with a dash of arcane casting (small but good spell list), a familiar, and the ability to curse opponents. This is an interesting class, a bit like an arcane, slightly evil-themed Paladin. Gets mettle, and Cha to saves against spells.
Swashbuckler - a finesse fighter who gets bonuses with light weapons and to Reflex saves. Also, better skills than a Fighter, but overall, a bit lacking. Is often considered a 3 level class (as it gets the only really worthwhile ability at 3rd level, namely, to add his Int bonus to damage with a light weapon).
Complete Divine has:
Favored Soul - what the Sorcerer is to the Wizard, the Favored Soul is to the Cleric. However, having to pick your divine spells on a permanent basis without spontaneous cure spells makes for some hard choices. Gets some outsider-like resistances. Not as powerful as a Cleric, but lots of f(l)avor.
Spirit Shaman - he casts Druid spells with a weird spell selection mechanism (pick a spell list in the morning, then cast those spells spontaneously for the day), gets a spirit companion, and is very good against 'spirits' (Fey, Undead, Incorporeal stuff...). Flavorful, but a little hard to understand at first.
Shugenja - elemental/oriental themed divine casters with very restrictive spell lists (choose one) and few class features beyond that. No armor and weapons mostly, so this one loses out to straight Cleric. Flavorful, but hard to make work, like all the new stuff here.
Complete Adventurer has:
Ninja - oriental themed Rogue-ish guy. Can use ki powers, most importantly the ability to turn invisible for short periods, and gets nice skills. Overall though, there's nothing the standard Rogue couldn't do as well.
Scout - somewhat of a crossbreed between Ranger and Rogue, the Scout gets the unique skirmish ability (deal more damage and AC bonus after moving), as well as a fat skill list and other goodies. Nicely balanced, and fits with many outdoor builds.
Spellthief - an arcane guy/Rogue who can make a special attack to steal a real caster's spells and use them for himself. Can also cast a very few spells himself. Rather pigeonholed all in all.
Complete Arcana has:
Warlock - fiendish themed blaster guy who can use a very limited selection of spell-like abilities at will. Not really flexible, but has lots of staying power and can always fall back on Eldritch Blast (damage plus optional effects). Also gets some outsider-like resistances. This is something original at last!
Warmage - cross a Sorcerer with an Evoker and you get this: a pure blaster with lots of kaboom power and little else. Can also wear armor and cast. Is often described as an "archer with a larger special effects budget", but will be appealing to many.
Wu Jen - oriental themed wizard-like arcane caster with a different spell list. Like the Shugenja, elemental-themed, but not as restricted by choice of element. With more support, these guys could have been interesting. As is, you can just play a theme Wizard instead. More spells for them are in Complete Mage, at least.
Player's Handbook II has some of the best:
Beguiler - Int-based spontaneous caster who relies on illusions and enchantments mostly. More skill points than most other people, with a good skill list, means he can double as skill monkey. Casting in light armor and a few casting tricks makes him less squishy. Excellent design, not overpowering, but interesting.
Dragon Shaman - this guy venerates dragons and uses magical auras to grant himself and his allies some of his idols' powers. He also gets a few draconic features himself, including a breath weapon usable every 1d4 rounds. Not that fantastic a damage dealer, but great party support.
Duskblade - the real gish class, this mixes melee fighting with arcane casting (up to 5th level spells off a small, but good list). Can channel spells through his melee attacks and deal obscene amounts of damage that way. Flavor this however you want, it just works.
Knight - almost forgot about this one. It's a tank with un-special abilities. He can call out single opponents who are then forced to fight him, he gets good proficiencies and Hit Die, and a few haphazard bonus feats. All in all, small wonder I forgot about the Knight...
Tome of Battle (the Book of Nine Swords) [if you don't know what this book is about, you'll probably not understand what I'm saying here; it introduces a wholly new mechanic usable in melee]:
Crusader - like all ToB classes, the Crusader uses maneuvers (once-per-encounter powers) to make himself the best tank there can be. Also has a damage-delaying effect and can reroll saves for staying power. Uses the Devoted Spirit, Stone Dragon, and White Raven disciplines.
Swordsage - a battle trickster par excellence, the Swordsage has more maneuvers than anybody else and gets some tricks related to specific disciplines over time. More of a light fighter overall, you can basically make him do whatever you want. Uses the Desert Wind, Diamond Mind, Setting Sun, Shadow Hand, Stone Dragon, and Tiger Claw disciplines for great versatility.
Warblade - maybe the most powerful melee class there is. Warblades are excellent at 'going over there and killing that'. Use intelligence more than other fighting types, and have access to the Diamond Mind, Iron Heart, Stone Dragon, Tiger Claw, and White Raven disciplines.
Tome of Magic [introduces three completely new mechanics of magic]:
Binder - the binder binds 'vestiges' (fragments of lost souls) into his own soul to gain their powers; these are usually a set of supernatural abilities that are derived from the individual vestige's history and personality. Over time, they learn to bind more than one vestige at a time. Some vestiges grant combat power, others make you a good diplomat or allow Turn Undead, using magic items or whatever. Great flavor, great versatility, but very hard to get to do what you want - you really have to know the game's mechanics and the vestiges' powers well to make good use of them.
Shadowcaster - this full caster uses shadow-based 'mysteries', which sounds flavorful, but doesn't work so well in practice. He has very little staying power and can use his best mysteries only 1/day. Over time, his lesser mysteries become spell-like, then supernatural abilities, which adds a nice touch. The mysteries themselves are a little underwhelming for the level, really. I'd play a shadow-themed Wizard over these guys any day of the week.
Truenamer - this guy uses the power of the true names of all things to manipulate them. Unfortunately, the class is designed so poorly that it simply doesn't work. Forget about this one. Seriously.
Dungeonscape has only one, but it's kickass:
Factotum - the Factotum receives lots of Int-based abilities, which overall let it do anything it needs to do competently. You'll not be the greatest fighter, or spellcaster, or healer/Undead turner ever, but you can do those jobs when needed. You can also add your Int bonus to basically everything once in a while, and your skill list simply reads: 'all'. Is that awesome or what? The best thing is that, in my opinion, the Factotum isn't even overpowered - just competent, and never without something useful to do.
Heroes of Horroralso has only one:
Dread Necromancer - like the Beguiler and Warmage, this is a spontaneous caster who's specialized in a field of magic: you guessed it, Necromancy. He also gains a scary familiar and turns into a lich over 20 levels. With cool and spooky abilities, the DN can compete in melee at early levels and lead undead armies at high levels.
