Highest killstreak was in Warzone standard on Darkstar a few months back. I had gotten to the upper 30's in kill count using a Phaeton Helios, but got taken down due to an Endgame shot I couldn't dodge. I'm not so much proud of that occurrence so much as the most recent encounter I had in Reach.
It was a BTB match on Boneyard. I was going ham in the Banshee and my teammates did a great job of locking down the main play area (where the scorpion and SinoViet center are), so much so that we ended up effectively spawn camping them. Predictably, most of their team quit out, except for one guy who refused to leave and was hiding somewhere. Since nobody could find him, my guys started betraying each other and then bailed too, leaving just me and him.
I took the Banshee to the other side of the map where he was hiding in one of those orange and grey shacks. I killed him a few more times to earn an Untouchable and my first ever Perfection in Reach before the game timer ran out. I feel kinda sorry for the guy, but props to him for not bailing like everyone else.