Oblivion, because of a couple reasons:
Short List Commence
1. It seemed like such a huge and detailed world. You knew you could never explore the whole thing.
2. You can easily dodge magic, arrows, and traps if you are smart enough to put points into Speed.
3. You can shoot lightning out of your hands.
4. LIGHTNING
5. Sheogorath's Realm, and Sheogorath himself, are so awesome they are almost better than the original Oblivion.
Yeah, actually, Shivering Isles is better than Oblivion. But Oblivion is the main game, so we'll call it that.
Short List Commence
1. It seemed like such a huge and detailed world. You knew you could never explore the whole thing.
2. You can easily dodge magic, arrows, and traps if you are smart enough to put points into Speed.
3. You can shoot lightning out of your hands.
4. LIGHTNING
5. Sheogorath's Realm, and Sheogorath himself, are so awesome they are almost better than the original Oblivion.
Yeah, actually, Shivering Isles is better than Oblivion. But Oblivion is the main game, so we'll call it that.