Mine: a workshop leader once told me to "remove every adverb on principle". I tried it for a draft and the prose became flat and weirdly accusatory — every verb doing too much work alone.
Adverbs aren't the enemy. *Lazy* adverbs are. There's a difference between "she said quietly" (which can be cut) and "the room ticked steadily" (which is doing real work).
What's the worst rule someone has tried to impose on you?