1) (tie) EverQuest and StarCraft
3) Halo 1/2 were addictive to me. Nowhere near the extent to which the first two were though. I would play Halo 1 multiplayer on every possible occasion.. Halo 2 as well. (Also, trying to beat Halo 2 on legendary became an obsession for me for a while)

AberMUD
Rogue/Hack/NetHack
Might & Magic 4+5 (combined to '9')
Diablo

Rhythm games like guitar hero are the most addicting for me, i play them almost everyday. Fallout 3 and mass effect where close seconds. And halo 3 used to be quite addicting before they changed the weapons and ranking ways.

Call of Duty 4: Modern Warfare

Gears of War. Trying to pwn pros while having insane shotgun lag is a brutal challenge.

Counter-Strike..ohhh bby. I didnt care much for PC gaming but that game changed me =P