It depends on what game mode you're playing...
For Deathmatch gametypes (TDM, 3TDM, FFA, Hardcore) Cloaked and Monkey man will work wonders.
Why? Cloaked obviously gives a huge advantage. It is a terrific flanking tool, and when combined with Monkey Man, you're a force to be reckoned with, since climbing is a HUGE part of the Uncharted universe.
For Plunder, I always use Beast Mode and Back in the Saddle.
Why? Well, Beast Mode is really only useful in Plunder, so that's why! 
And Back in the Saddle could be the deciding factor between winning and losing. The decrease of four seconds does make a significant difference.
For Team Objective, it's Endurance and Back in the Saddle.
why? Decreased sprint time and increased sprint duration are going to help a lot in OBJ, when you'll most likely be dying more than usual. Endurance will get you back to the hill, idol, marked man, etc. that much faster. And, again, the four-second-shorter respawn time will make a big difference with Back in the Saddle.
For Co-Op Arena/Adventure, I use Regeneration and Teamsafe
Why? Regeneration is no joke. It will heal you very fast, which will help you more than any other booster (in my opinion) especially when playing on Crushing. And Teamsafe, so you don't down your teammates!!
