I'm not sure whether you mean an assembler to use on an emulated Apple or outside of it. If the former, you can't go wrong with
Merlin (also at
ftp://ftp.apple.asimov.net/pub/apple_II/images/utility/programming/merlin_assembler/ ). If the latter,
dasm is pretty good. How to get compiled code into a state usable under an emulator is left as an exercise for the reader.