I generally prefer to play ranged DPS classes. My first ever toon in an MMO was a Wizard in Everquest (Scars of Velious era) named Grizlith. He didn't make it too far. My friends were much higher level, and back then it was damn near impossible to solo a wizzy at low levels.
After that, I went on to a string of bolt casters in Dark Age of Camelot. I played up a Void Eldritch, a Runekeeper (I'm spacing on the spec line), and a Fire Wizard. Trials of Atlantis murdered DAoC, and I stopped playing MMOs altogether for a while.
[Insert utter obsession with Phantasy Star Online into this interlude. Class of choice: RAcaseal]
Everquest Online Adventures came out for the PS2. It got me back into MMOs, and my first toon was (surprise surprise) a Wizard named Grizlith. A Hobbit Druid with green hair named Yeebo was also whelped there. Though I didn't play it for very long, my obsession with ranged DPS classes was well established during my time in the game. Had Grizlith been an effective solo toon, I would have played him straight to the cap.
EQOA is notable in that it provided quest based progression until around level 20. Back then quest based progression was unheard of. "Grind or die!" is how most MMOs handled leveling at the time. Even in EQOA, you ground your way up EQ style to the cap after 20. As far as I know, EQOA was the first MMO to clearly demonstrate that quest based leveling is a lot of fun. The clearest proof was that the game bit from 21 on (once the quests dropped off).
Back to the main story:
At some point Anarchy Online, SWG, and EQ II come into it. But the one that sticks out is WoW. And the main reason is that the Mage in WoW was the most skilfully designed ranged DPS I had ever tried in an MMO. Tons of damage at range, but you had to know how to mix long cast time and instant cast abilities to get the most out of it. Squishy, but with lots of neat defensive abilities to flee for your life if you got in over your head. It felt a lot like a bolt caster from DAoC or a Wizzy from various EQ games, but more dynamic. I was hooked . . . until I hit 60 and the game fell flat on it's face. It switched from "Everyone is welcome here :-)" to "Raid or Die!" within few weeks.
There is quite a bit after that I'll skip. Some of it may relate to this narrative, but it's hard to say. For example, I spent a lot of time playing various Blaster builds in CoH, and seeing how much damage a Defender was capable of if you specced them purely for offense. However, the next MMO that really sticks out in my mind is Lord of the Rings Online. As an instantiation of Tolkien's work, the game is impressive. And I am certainly a fan of Tolkien. However, I think that a lot of my obsession is related to how damn fun it is to play a Hunter in LoTRO.
The Hunter in LoTRO is an incredibly fun ranged DPS class. They do crazy damage on single targets, have a group runspeed buff, can teleport all over the global map at will, can lay down traps for crowd control, and can use tracking to find their targets faster than any other class could hope to. They sound overpowered on paper, but the fact of the matter is that, due to a lack of defensive abilities, they have trouble soloing encounters some other LoTRO classes can handle with ease. Powerful, but still challenging to play. Pretty much exactly what I look for in an MMO.