I played so many MMOs I can't even count them (dozens), but if I had to chose TOP 3 I would set aside as most memorable ones:
Vanilla WoW - my first MMO, the strongest nostalgia, most friends made through the years playing, amazing game until Blizzard nerfed everything down the line with each following expansion.
Allods Online - an amazingly designed game, immense amount of content, great class balance, super fun, best ship combat I have every seen, super fun PvE raids, GvG, open PvP. Just one HUGE minus - it is an IMMENSE P2W. -- (If P2W didn't exist, this right here might have been (overall) the best game ever designed).
The Secret World (ORIGINAL edition) - The best PvE game that ever came out. I'm emphasising PvE part, because PvP was only 3 mini games and that was it. PvE however was the TOP. Best story, most amazing quests, awesome raids, superrior theory crafting, game EXTREMELY based on knowledge and player skills (quite a few players actually quitted due to game being too hard for them). -- (TSW Legends expansion ruined the game completely, and game is currently trash.)