1. World of Warcraft
The highest production values of any MMO to date, WOW was great for Blizzard's ability to take what EverQuest and Dark Age of Camelot did well and combine them into a game of obscene quality. Though still flawed, WOW is my favorite MMO to date and was the first in which I really enjoyed PVE.
2. Dark Age of Camelot
Though it was plagued by balance issues DAOC was the first MMO to really get PVP right. Since Blizzard never did get around to replicating the best aspects of DAOC's RVR combat it remains my favorite PVP MMO to date.
EverQuest took the MMO aspect of Ultima Online and almost entirely struck the PVP aspect, pursuing a PVE game. If UO was the first hardcore PVP MMO, EverQuest was the first hardcore PVE MMMO.
4. Ultima Online
Arguably the first MMO, Ultima Online was the wild west pioneering days of MMOs. It was cool to see it at the beginning...