I got Oblivion as a high school graduation present. I had never played an open world RPG before, and this game blew me away. Needless to say, there went my summer. Unfortunately, the laptop I had for school couldn't handle Oblivion, so to get my Elder Scrolls fix I backed up and got Morrowind. Loved that game too, but the technical gamer in me repeatedly pointed out things that were tedious/annoying/just plain stupid in Morrowind that Oblivion fixed up. It was, perhaps, the clearest case I found at the time of a game whose sequel fixed the mistakes of its predecessor and surpassed it in terms of quality. I love Morrowind and think it's a great game, but Oblivion is better.

And I haven't played Skyrim yet. I'm looking to fix that soon-ish.