My assembly programming in order: 6502, 6809, 68000, Z80, x86, ARM.
ARM was my favorite (doing low level stuff for gameboy advance development), it had some interesting features like switching between arm (32bit) and thumb (16bit) modes based on whether the destination address of a jump was even or odd. Followed by 68000 (on my amiga), so much nicer than x86 (especially early x86 with the memory segment crap).