The first member of my fantasy Avengers roster is...
The Mighty Thor.
I couldn't think of a more marvelous power house that comes from the House of Ideas. There's might, magic, and majesty. Sure the JLA has Superman (just not lately), and to think you have a god on your side probably boosts your ego a bit. I would imagine so, anyways. Thor's book now is simply fantasy as it's best and I also couldn't think of a better leader of my team than the Odinson. He's been seen in numerous versions of the Avengers team, also serving as a founding member.
It's easy to see why I would choose the God of Thunder here.
(Art provided by George Marston of If This Be Sketchblog fame, and Heir to the Realm of Majesty)