Transport Tycoon Deluxe (because I've been playing it on and off since lauch and doubt I'll ever tire of it)
Command & Conquer: Red Alert 2 (same reason as above)
TES: Skyrim (because random quest generator and SNOW)
Non of those games are my favorite, but i think MMOs have the best possibilities to change over time. I've put Dota 2 in because its a differnt from the other 2, and hopefully will have a long life cycle.