The Elder Scrolls 4: Oblivion simply put, is one of the finest games ever made. Not many games in the RPG genre carry a perfect blend of RPG elements like character interactions, main story, open world, and variety of quests like this game does.
Beginning with the plot, this game deserves a bow. What makes this even better is they way it is executed. The voice actors really have succeeded in putting life to each and every character, excluding you (Who unfortunately does not speak). All the quests are inter-connected and pull you into the game - Making you unaware of your real life as you play it.
When variety is concerned - I personally haven't seen any game that is loaded with so many quests, creatures, items and spells. A 100% completion of this game seems like a myth as there is always something you can do. And this is because the game has a huge open world for exploration. You also get to be a part of various guilds which have an individual story of their own. However even after all this variety - The game has too many cave/shrine levels which are rather frustrating and boring after a certain period of time.
Believe me, just after you start this game - You are going to be glued at your seat for weeks. This game can really be a torture for your eyes since you will never feel like closing it. It is a must buy for everyone - Not just RPG lovers.