AC-130, the predator missile, and the reaper are the most satisfying streaks to get kills with I think. The black ops 2 streaks a bit lackluster in that way but at least they're very useful, though some are too good.
^This. It was way more satisfying calling in a moab in mw3 than getting a nuke in mw2. I always thought it was beyond stupid how you can get your harrier and then let your streaks get you kills just to kill yourself and everyone else and end the game.