My fave of all time was Asherons Call 2, even at cap with Hero status, the game; with the right people, just didn't bore at all. (R.I.P)
I've also had a blast on the original Asherons Call, DAoC and Everquest, but alas, came to MMOs too late, and missed their peaks. Still enjoyable, from what I experienced. I would have loved to have been with AC from the beginning.
WoW ofc, recently left due to End Game content being a bit too easy. I wasn't expecting to leave my first few runs of Nax 25 with half the tier7.5 gear... Nor clearing the entire place! Where's the challenge?
ATM I'm back on LOTRO. Left a while back when Angmar was just boring me rigid, found a new lease of life with the Warden Class. I would however recommend you play with a group, or duo. So much more can be accomplished doing such than trying to solo the content. I'd dare say if you tried Soloing the game, you'll quickly get bored of it's slow progression. I play on Snowbourne, and currently unguilded, so if any kin needs a new player.. /w Failrien or Ariael.
Others I've played are City of Heroes/Villains, Everquest 2, Vanguard: SoH, Guild Wars, EvE and WAR. None of which, with the exception of CoH/EQ2 grabbed me enough in the latter half of their progression to keep me with them.