My Killstreaks change depending on the gametype....
Hardcore S&D: UAV, Predator Missile, Care Package/Harrier Strike
I try to keep my streaks low for S&D because most of the time you won't get above 4 or 5 kills before someone pre-fires and gets you, or tubes you, or you slip up. I also switch back and forth on the Care Package and Harrier Strike. Because a lot of times I can get that 7th kill without dying. And most of the time a Care Package is crap anyway (ammo, a UAV... or even more worthless for Hardcore S&D.... a Counter UAV).
Free-For-All/Hardcore TD: Predator Missile, Harrier Strike, Chopper Gunner
I think this is a pretty good setup. You get 5 kills and you get your Predator Missile, 75% of the time you can see at least one person to hit. 1 more kill and you get your Harrier Strike. You can hope that you get at least 1 person with the strike itself, and one person from the Harrier. So now you're only 2 kills away from a Chopper Gunner. You try not to rush out there and get yourself killed, and you should get 2 kills. And then you get to fly around shooting people.