My highest is 68-0 in a domination jungle ground war game. So technically that's infinity! (jk) Nothing really special that I did. I used BB/CG/Gunship. I kept the BB until after I used the other two which they had a hard time shooting down.
There is a guy I play with who's name is something like Jumbo Jezus and his highest killstreak is 118 from a ctf game on Nuketown. I don't know wtf he did but that's pretty crazy. When someone had mentioned that his highest KS was that I figured they meant including using killstreaks(like mine with chopper gunner) but apparently not as you can see it on his combat record. The guy isn't a booster as he's really good but I still don't know how he did it.