One of the best games ever made. Period.
Oblivion is an RPG. You have your usual RPG fare, but the greatest selling point in Oblivion is its size. The world is HUGE. I personally have not gone everywhere and there is a lot of places I have yet to explore. There is a lot of things to do. You can be an assassin, a mage, an archer, a thief or whatever. You can be all of them at the same time!
You can spend hours just walking around exploring the world. The game encourages you to do what you want to do. You can go around wandering aimlessly from the start and not get penalized in any way! In fact you might even get rewarded for doing so!
The story is also top notch compared to other games of the same genre. The story is immersive, you really feel like you are inside the game. I'm not going to spoil the side quests that is a large part of the game as they are worth experiencing for yourself.
There's not much to say about this game as any gamer would recognize this game as a legend. You'd be pressed to find a game like this. Oblivion is truly a rare treat.