I think I can out-oldschool most of you.
[You must be logged in to view images. Log in or Register.] Not strictly MMOs, but all in the same lineage imo...
Zork I, II, III
Ultima III
...
D&D (yeah, moster manual, all that stuff)
Hack/NetHack - classic!
...
Diablo
Diablo II
EQ starting 9/99, and now still going in P1999!
AC (beta)
VG (beta)
WoW
Favorites...
1 - EQ
2 - D&D
3 - tough one - probably about a 10-way tie