Given the scant details you've divulged so far... there isn't one that's "best." Pick one. Merlin is probably the one that is the closest to a defacto standard.

But if you're just wanting to dabble, your IIe has a mini-assembler built right in. Just CALL -151 from BASIC, then hit the exclamation point to get into it. See:

http://www.atariarchives.org/mlb/chapter3.php