Lot of love for Techmo Super Bowl on here. We used to have tournaments on a Fri or Sat night in the dorms. Everyone knew to take their Oilers before I got the chance to take them or I'd roll them. Randall Cunningham was a cheat code in that game too.
My five in no particular order then...
Techmo Super Bowl
Red Dead Redemption
NCAA Football, esp the last one since I love running the read option (cover not so much)
World of Warcraft (even if they have dumbed the game down tremendously since the early days)