If you start with assembly I would suggest not picking x86. I would suggest a RISC or virtual assembly like LLVM. x86 just has too much historic weirdness.
Like scores of other generation C64 kids by second language (after BASIC) was 6502 assembly. I think it's suitably straightforward. Although the special addressing mode for the zero page might count as a weirdness.
On the other hand, the x86-64 is a lot nicer and you certainly don't have to go very far to find one to practice with.