My favorite streaks are the Stealth Chopper, Warthog and VTOL . I honestly think the Dogs and Swarm are way to overrated. Yes, they are cool, but they aren't worth going for at that price. It's far to easy to kill the Dogs in my opinion and I usually only get 4-5 kills with them. The Swarm is like the Hunter Killer, if the other players have the right Perks on you won't get a lot of kills with it either. I usually get about 6 kills with a Swarm. The Warthog on the other hand, I think is underrated. If you haven't seen one or heard one, you are missing out! It is the coolest one in the game! I get at least 15 kills with it every time. Stealth Chopper is one that will either do work or do nothing much. The thing is that it's easy to get and you can kill the guys trying to take it out too. Usually, if you get the Stealth Chopper you are a lock for the Warthog, which in turn gives you an easy VTOL. Yes, people can use Perks that make it hard for you to see them in the VTOL, but you are the one shooting and can usually rake in kills with it regardless of what Perks the other team uses.
If I'm playing with my crew, I usually use lower Streaks that are more for defense around objectives. I'll either use a UAV or Care Package then Guardian and Sentry Gun.