I'd say Sabretooth. Mainly to get him out of the X-books. lol The last 2 years, his best characterization came from non-X-books. But in general, that seems to be the case. When Rogue was an Avenger, she got lots of shine. Same for Sunspot. Now they're both back "home", and have done nothing since then.
Monet could probably thrive on the avengers as well. It did wonders for Sunspot & Canonball.