Man, I had to think way too long to come up with this list. Notably I excluded WoW simply because I never put an extended period of time into the game, and think my pessimistic view of the game coming from someone who played every other title up until 2009 before even trying it clouds my judgement.
First, Runescape. I was watching a stream the other day and the streamer asked what everyone's first MMORPG was (7k+ viewers), and Runescape flooded the chat. It's a game that introduced so many people, myself obviously included, to the genre and kind of defined the genre. Upgrade-able gear which can be found as drops or learned through the progression of crafting, professions which have meaningful use with quests, a story that follows a direction but isn't completely linear nor required for the most part, a map that I can remember almost perfectly 10 years later, etc. I have been searching for my "next Runescape" ever since I quit, and that pursuit is really what drives my interest in playing any/every new MMO.
Second, Maplestory. I think a lot of people harbor resentment for a game as popular as Maplestory just by nature (same goes for WoW), but Maplestory gets a lot due to it's overly-grindy nature. To me, the game just had this charm and sense of community, with features that rival that of the best games in existence today. I could hop on my friends and play for hours and despite the game being boring as hell alone, playing with friends made it so enjoyable. Switching channels instead of servers was a nice touch to help enjoy content, but there were communities which formed on specific channels. It sparked so many amazing forum communities, Youtube content creators, etc., and that's why this game still sticks out as one of my all-time favorites.
Third, Guild Wars. I hesitate to mention it here because I will often say it's not a true MMO, but I think it's place in the list is warranted if only by the spawn of Guild Wars 2 and the systems in place which mimics those of many MMOs today. Guild Wars to me was the perfect way to have an MMO storyline, although the instancing of everything was a little over-the-top. Additionally, the PvP and battlegrounds are still at the forefront of MMO PvP in my mind.
Fourth, Rose Online. This one was tough but I think it represents a great example of the shift that has occurred from a P2P-centered MMO universe to the current F2P-centered one. Rose was a great game at the time which was incredible when playing with friends, but plagued long-term by competition with private servers offering free play. Had it been F2P from the start, I think this game would have had much more success, as it truly was a fun game.
Last, Aion. I think Aion took a huge stride forward for MMOs in general, providing a really top-notch gaming experience with polish added to old systems and the introduction of newer ones such as flying-combat. The open-world combat in the game was top-notch and some of the most fun I have had in that regard, despite never reaching level-cap due to the horrendous grind which the game suffered from at launch. I've heard good things about the updates to the game now, and had the game not had such a grind at the start I may have been so hooked I would have still been playing it today.