I think it goes without saying that everyone has a preferred class to play when looking at an mmo.  My favourite classes were always classes that could heal themselves and deal damage at the same time.  The Disciple of Khaine from WAR was probably one of my favourite (basically the only reason why I kept playing that game).  Guild Wars gave me the choice to basically make up my own class, which I had a ton of fun with.  15hp MO for the win.

But going back to oldschool games, I think I had the most fun on my Monk in EQ.

What were yours?

My Archer in Asheron's Call. AC didn't have classes, allowing you to pick a set of skills and go. I like it when I have access to all skills in a game and can define my archer, swordsman, Dagger fighter, etc, as I think and not what devs tell me it should be.

Rogue class in EQ and the bard class in EQ and Vanguard. Monk class in EQ is pretty fun too.

My Nox Dexxer in UO, followed by my Resto Druid pre expansions in WoW.

probably the Vanguard sorcer .. he levitate man!... or the Warg on LOTRO, or my "spy-engineer" on planetside (that is not a class, but a build, but anyway...)

Shadow knight from EQ. I was the first SK to successfully solo the froglok king waaaaaay back when. And no one wanted to mess with a high level SK on PvP servers. Heh.

The Warden in LOTRO is my favorite now.  Leather Armor.  Small self heal.  Sword and board.  Javelin for range.  Doing the main skills in a certain order leads to different combo finishers.  The cycle timers are quick enough that you always get to push a button, which for me is huge.  Best manage of skills and abilities that I've ever seen in a mmo. 

The tauren hunter in WoW comes second just because of the finger snap when doing hunters mark.

Cleric in Vanguard or maybe the Assassin.

FFXI Samurai.  Loved that class.

Druid in WoW.

The Mentalist from daoc.  He can heal, dot, nuke, hot, mana regen, ae nuke, stun.  The best caster ever.

Anarchy Online Engineer.   Nothing like have a HUGE bot to do your bidding.

Creature Handler - swg

Originally posted by John.A.Zoid

Druid in WoW.

I second that. I loved being feral. I could tank and DPS with the same talents, with only a few equipment swap-outs.

Rouge in wow, lolz.

jedi and BH before cu

A necro Mastermind from City of Villians.

Necromancer.  Started in EQ1 and still can't forget it. Always looking for something similar in all MMOs.

Pet, massive Damage over Time spells, snares, kiting... Life drains...

Everquest 1 - Necromancer - Best class ever made, it had everything, great group spells, great solo ability.  The perfect package.

I have a few;

EQ - Necro/Magician

UO - Tamer

AO - Engineer, Crat and Meta

SWG - Creature Handler