Sona and Soraka, no contest. Because apparently just because someone isn't getting kills they are rarely focused by allies in a teamfight, and my team rarely seem to realize they give their allies free stats and a bunch of health. I swear both their ultimates generally go unnoticed by the enemy.
Common sense seems to be more uncommon. League of legends sig: http://www.leaguememe.com/page/meme/id/3467/sort/all/type/1