Chopper gunner. You get more kills off of it than any other one, and its only 11 killstreak to get (which isn't bad considering you get at least 10 more from using it).
Chopper gunner is great for building your killstreak as when i was Karachi I got like 10 kills with it and it I don't even have it as a killstreak I found it in one of those crates lmao. I can only imagine causing havoc on scrap yard with the chopper gun.
I only have a few and the Sentry Gun is freaking beast!
I can't get the sentry gun to do Jack, I've gotta be doing somethig wrong. I planted one staring in our hq, all the enemies just walked in and took it, gun never even shot.
After that I took it off and replaced it with harriers! Lol
I've gotta say my fav so far though is chopper gunner! Raaaaaaaape!