UAV, Care Package, Attack Helicopter
UAV is just an amazing killstreak and gets me more kills than any other killstreak. I've also noticed that anything you get in the care package is useful (no ammo crates) and I've gotten a couple of assault juggernauts, and osprey gunners for it. And unlike MW2 or Black ops the attack helicopter is a solid killstreak that does its job properly.
PS: This is info is coming from a 3.1 K/D player