Master of Magic. I am a huge RTS and TBS freak. I've played nearly all of them I could find, and this is the one I keep going back to. Even given that Age of Wonders is a blatant rip off, I still play MoM instead. Although, Dawn of War was pretty damned awesome and I will play any expansions/editions they make for that.
The original Pool of Radiance (all the 'gold box' adventures for that matter), Wizardry, and Bard's Tale (1,2, and 3) Every now and again, I get this sick NEED to destroy critters while wondering about perfect grid mazes. Even with Etrian Odyssey, Orks and Elves, Mazes of Fate, etc out for DS/GBA, I will pop these back in (thank you DoxBOX!) and stomp already well stomped ground.