I had an Atari 1040 ST and I used to love programming it in 68000 assembler mostly using K-Seka. It really felt like programming a 32 bit CPU which was amazing after the 8 bit world that preceeded it. The instruction set was very orthogonal (other than the slightly odd split between A registers and D registers). When I graduated from 68000 assembly to ARM assembly it felt like a natural progression.

Probably my best project was a FORTH system. It used direct threading so each FORTH word was a proper assembly routine. It had primitive peephole optimization too! It was all written in 68000 assembler with K-Seka.

So if you want an assembler to learn, 68000 is a great choice. However you could learn 32 bit ARM which I came to prefer and that will still run on modern systems (at least if they have been compiled with 32 bit support or on the plentiful ARM microcontrollers).