In Online matches, I never go for higher Scorestreaks as I'm constantly on the losing end of lag/lag compensation, so getting them is virtually impossible most of the time, forcing me to stick to lower-tier Scorestreaks.
UAV / Hunter Killer / CUAV is what I usually use online.
Only can get higher ones playing against Bots =\
If I go by favorites alone, then I'd go with A.G.R. (or Warthog on more open maps), K9 Unit, and Swarm.