Oblivion: Gameplay and graphics were a huge improvement over Morrowind, and I found the entire thing entertaining. I've gone through the Dark Brotherhood quests 4 or 5 times now. I think it deserves some recognition.
Fallout 3: It was like Oblivion with guns at first, which kinda turned me off tbh. But then I truly played it. The story was great, the universe is rich, and the guns were fun. I didn't like how stealing negatively impacted karma, which was my bread and butter in Oblivion, yet it did so many things right for me. All-around good RPG.
Galactic Civilizations 2: Fun game. Whenever I grow tired of the other games, I always return to GalCivs2. I usually run a marathon session, seeing how far I can get in a certain time limit on the largest universe with the most habitable planets and enemies. When I get tired of the huge, galaxy-wide consequences of my actions, I have some fun building a fleet that is named and designed in such a way as to imply uniformity. These ships were designed to fly together. Then I unleash them on the unsuspecting masses.
Evil Genius: Lesser-known, but still fun. Comedic, eccentric, and a fairly unique take on the genre, this is another of those games I go back to when I grow bored of the others.
Battlefield 1942 (And various mods): Perfect LAN game. Dice and EA did an excellent job with this game. The customizability, the hilarity of some moments, and the simplicity of LANning it all make this an excellent candidate for any top games list. Highly recommended.
Things I didn't include: Fallout: New Vegas, KOTOR 1, Diablo 2, Dungeon Siege 1 & 2, Civilization 4, Battlefront 1&2, CoD 2, Rome: Total War, Majesty 1 & 2, Magicka, Just Cause 2, GTA 3/Vice City/San Andreas, Borderlands, Starcraft, etc.