For anything written from scratch, I would recommend fasm or nasm.

I prefer the latter, because the documentation is better and there's a way to specify target cpu (e.g. 8086) and get errors when instructions aren't compliant.

I agree - nasm is excellent. I've used it for pretty much all my MS-DOS projects (games and demos).

Ah yes I forgot to mention both tools are also still under active development. I mostly use JWASM to compile samples from old MASM books/tutorials.