There are several.
One of my favorite games of all times certainly is good old Dungeon Keeper, I just keep reinstalling that thing on every system that I have that will take it.
X-Wing alliance also got it's fair share of playtime. Some of my favorite story driven games are Jade Empire (bioware, duh) and Jedi Knight 2.
Then racing games... The old Grand Prix series, mainly Grand Prix 2 and Grand Prix 3 are what got me into racing games in the first place (followed by Need for Speed 3), but the best I've played to date have got to be SimBin's GT-R 2 and Project Cars 2. I've also abused the hell out of Forza motorsport 3.
The Street Fighter series also keeps returning, my favorite definitely beeing Alpha 2 on the good old SNES (still got one working just for street figher and mario world).
and something I dabble in from time to time is Pokemon whenever there's a new generation out.
The Witcher 3 might make it into this list, but I've just started that game.
So, if I had to pick.
-Dungeon Keeper
-X Wing Alliance(or Rogue Squadron)
-Jedi Knight 2 & Jade Empire
-Street Fighter Alpha 2
-GT-R 2 (& Project Cars 2)