Whats the best gun ever?
Sorry- we can't attempt an answer until you tell us best for WHAT. A rifle that can hit targets at 1200 meters is useless for bird hunting, and very difficult to carry for personal defense. Your question is far to broad to be answered briefly.
There is no "Best" Airsoft gun, but the strongest type is Gas. IF you want an all around gun though, there is an electric one called, the the g36k (i know what im saying, it's not a g36c)by a brand Jin Gong. Put an acog on it and you got a pretty far range gun with a 450 mag. Also if you by a M4 and a bigger mag it would work too.