Carbine :p DMR and BR are both better than the pistol, but its close enough that a pistol has a shot to beat them if someone misses a shot. There's a magic range with the carbine though were no other precision weapon has a chance.
I prefer the BR in arena because of the mid range smart link and .033 faster time to kill. But in warzone I pick the DMR because warzone are big maps and I need a long range scope. There is a long shot BR in warzone but I like the way the DMR feels at range.