If budgetary considerations don't come into play, I would say just get Skyrim. As the latest and greatest in the series (or so everyone says), you're getting the best version of the Elder Scrolls experience.
If money is a concern, consider getting the "Game of the Year" edition of Oblivion. The game still holds it own against anything that's been released afterwards. It's $20 via Steam, even less when it's on sale, and easily offers enough content for 200 hours entertainment--in a single play-through. If that's not enough, there is so much user made content out there that Oblivion could be the only game you play for a year.