Oblivion is truly a masterpiece. If you want fun, options, adventure, action, and depth, then Oblivion has it all.
The main story of Oblivion goes something like this: you are a prisoner, for whatever reason, who is freed by the Emperor of Cyrodiil, a land within the Elder Scrolls world of Tamriel. He indicates that you will have to help save Cyrodiil from destruction. Ultimately, the game is about you and your allies having to close gates to a hellish realm known as Oblivion. Of course that sounds pretty cool by itself, but the real greatness of the game comes from all the other stuff you can do. The main quest is only a fraction of what there is to be done in this game. You can explore countless dungeons, fight in a Roman-style coliseum, join guilds, become a vampire, become an assassin, become a thief, buy houses, and much more. Overall, the sheer size of Oblivion is overwhelming. But, what more could you ask for? You could spend well over 100 hours playing Oblivion and not get tired of it. I don't know how long it would take to complete everything, but you sure won't get it done easily.
Oblivion gets a perfect score from me because there is nothing wrong with it, in my opinion. There is a great story, brilliant characters, nice voice acting, good battle system, interesting level-up system, tons of customization, tons to do, great graphics, and sheer fun!
Oblivion gets a 10/10