Perhaps not a classic but one of my favorite games that I always come back for several times a year since 2004, is Evil Genius. Your goal, take over the world
Construct your secret, evil underground lair and build traps to protect it from the Agents of Juctice.
You never get bored (ok, the beginning might be a bit slow) and once you get your research started you just have to play 5 more minutes, then 5 MORE minutes etc. etc.
The humor in the game is absolutely great. Everything from the odd interrogation methods to the excellent voices of your henchmen. You find obvious parodies everywhere, like the Super Agents, John Steel (James Bond), Dirk Masters (Rambo) and of course Jet Chan (a Jackie Chan-Jet Lee-Bruce Lee-hybrid).
I've probably played through this game at least 5 times, which is quite a lot since the campaign is loooooong.