OG Starcraft. Still play it to this day, though in spurts. I'll play it hardcore for like 3 weeks, then stop for months.
Same with Rome: Total War. I'll finish a long conquest that'll last a few weeks, do some battle scenarios and take an extended break.
Quake series, mostly QIII: Arena
Elder Scrolls Oblivion and Skyrim
Fallout 3 and NV
(people probably hate me for only getting into the Bethesda life span of this series)
Halo 2 MP, Halo 1 nostalgia.
Super Mario World, and Super Mario 64.
Can't forget about Half-Life. Still waiting for number 3.
I game a lot, but those are the ones the stand out when I think about it.
It also goes with out saying, this fall is a big one for me. 3 of my top series of all time are all releasing new installments (Halo 5, Fallout 4, Starcraft 2 LOTV)!!