Oblivion - The Dark Brotherhood

The Dark Brotherhood quest line will give you a lot of decent items as well as a lot
of experience. To become a member of the fighters guild, simply kill a random character, and then sleep. A Dark Brotherhood member will visit you with a proposal. The fighters guild quest line also leaves you with some decent items and a lot of fighting experience. To become a member simply talk to your local fighters guild leader. You will find that the Fighters Guild will not accept you if you have a bounty or if you have a bad reputation.