I recommend also the 6809 as an example of another very well-designed processor from the same era. Most notably, it has post-increment and pre-decrement addressing, and basic 16-bit arithmetic support.
I, like you, started on the 6502 and then learned the 6809 (well the book was 6809 but the processor was actually the Hitachi 6309). They were fun processors to program on and I guess if push came to shove I would chose the 6809, but I had the most fun with the 6502 (actual screen vs. serial port).
I later learned IBM 370 assembler (why not have the CompSci's assembler class on a mainframe) and 8086/88. I hated both. They felt wrong compared to the 6502/6509. Although 8086 assembler did help my grade in the graphics class.
Assembler is great to learn because it teaches you what the final form of your program is.
About two years ago I picked up 6809 assembly while writing an emulator for it, to assist in reversing a device built around a 6809. Amazingly simple processor/architecture.
I later learned IBM 370 assembler (why not have the CompSci's assembler class on a mainframe) and 8086/88. I hated both. They felt wrong compared to the 6502/6509. Although 8086 assembler did help my grade in the graphics class.
Assembler is great to learn because it teaches you what the final form of your program is.