Most Amiga software was written in assembly, or in C (with inline assembler) using the old venerable Lattice C compiler set. The Amiga custom hardware really helped, once you knew how to utilise it.
These days the best assembly language to learn is ARM, in my opinion, if you want to use it in your career. For a hobby you could learn 6502 or Z80 8-bit assembler, then 68000. It gives you a good understanding of what happens on the metal.
