It would be any of Duke Nukem 3D, Oblivion, WoW, Warcraft(2 or 3), NFS Most Wanted, Diablo, Starcraft, God of War, Dawn of War, Spore, and/or BorderLands. However since you want just one, I would have to say Duke Nukem 3D. While all those games are great, I have been playing Duke Nukem 3D probably since it came out and I have enjoyed every minute of it.
1.BioShock 2 ( entire idea of game, deep ideas in there, ending twist) 2. RDR ( just good all together, awesome twist at the end ) 3. Dawn Of War ( one of the reasons i'm such a strategy fanatic) 4.Spore ( wuvved the whole game! i really had pity on the aliens though when the Grox took over or i blew them up with my massive laser XD ) 5. Total War Series; Rome, Barbarians, Medieval, Empire, Napoleon ( THE reason i love strategy so much. first encounter to strategy, good game, and became very addicted) 6.