Depends on the Pokemon you're teaching it to. If the Pokemon has weak Defense, than teach it Will-o-Wisp for the Attack crippling. If otherwise, Toxic is a better choice in every way.
EDIT: WOOHOO! I just hit my 100th answer! Clear your calendars, guys! Moomoo Milks are on me tonight! (I've really gotta cut out these Pokepuns. :3)