GS2
GS2 is a golfing language inspired by J and machine code. It was probably the first golfing language that uses a 256 byte instruction space.
GS2 is an extremely competitive language, even though it was created in 2013. Furthermore, some of the ideas first introduced in GS2 (Number shorthands, rest-of-program map) influenced several stack based golfing languages that came later.