Hmm, tough one. I supposed I'd go with whatever superhero/heroine my powers complement. i.e. If I had the power to manipulate the wind, I'd be Shayera Hol's sidekick, as I could twist the air currents to enhance her flight performance, or if I had the power to create shields around people, I'd go with the Flash as to grant him temporary invulnerability so that he could just bulldoze into opponents without risk of clothes-lining himself.
The only heroes I wouldn't sidekick for, however, would be Booster Gold and Batman. The first's a cocky douche and the second has a bad history of dead/injured/mindfucked sidekicks.