Never one to pass up a decent meme, I've compiled a list of my own top five D&D classes. I'm pulling from 3.5 (and related third party supplements) because that's the edition I've got the most experience with, as a player and a DM. These are more or less in low-to-high order, though the specifics would likely shift if I did this again in a couple days.
5. Cleric For nostalgia's sake, more than anything else. I've played a lot of clerics; it was kind of my spot in the first group, and I still tend to pick them, just out of habit. I tend to play them slightly loopy -- my crowning achievement being the Unitarian Universalist cleric (dedicated to a cause, of course) who had a nervous terror of pitchforks and torches.
4. Oathsworn And now, we get into the territory of "things I've never played, but would like to some day." I don't play much. But if I did, Oathsworn (from Arcana Evolved) would be close to the top of my list. I dig the whole oath/loyalty thing, and while that's not enough to hang a character on (as I learned in a short-lived Risus campaign) combined with some actually functional unarmed combat mechanics, you've got yourself a fun time.
3. Shadowcaster I'm mostly just all over the flavor on this one. Dark, shadowy, and creepy, without descending into the angst-fest that is the Warlock. These guys chose their powers. Add in all the extra goodness in the shadow magic section of the Tome of Magic, and you've got a class that I'd seriously consider using on the other side of the screen, as a villain or unreliable ally.
2. Psychic Warrior I dearly love the 3.5 psionics system, so much so that I'd consider running a campaign with it as the major power source, little or no magic. And I particularly dig the psychic feat trees -- they open up a whole bunch of fighter-type powers, but with a supernatural flair.
1. Champion I like Arcana Evolved. A lot. Champions, in particular, always struck me as an interesting and playable twist on the paladin -- a class I dearly love, but that has caused me more headaches than anything else. Champions are not one class, but several, each devoted to a different cause. ("Freedom," "Life," "Justice," "Death" and so on.) Lots of room for new material, and lots of room to tweak a concept that's not quite working, since switching causes is fairly easy, up to a point. (Like if you're running a Champion of Justice, and the rest of the group decides they want to be pirates . . .) Add in a lot of really interesting different powers for each cause, and no requirements for your companions behavior besides "help me fight these people who don't like my cause," and you've got a class that's great in theory and in execution.