I play Solitaire & Minesweeper rarely. Well, any games rarely nowadays.
Games I've been playing most though are Roller Coaster Tycoon 3 (with
Soaked! expansion) and Final Fantasy 7 really late at night when I have the time and patience. I completed FF7 in a week with no codes and no game overs just by playing late at night and none during the day. It isn't my first time beating it though.
Look out for
Discordi though. It should soon (hopefully) open back up for it's beta testing. I played it in beta during it's very early stages of development last year, and it was extremely addictive.