Gee, a few bites I would have voted slugs... but the entire flower??? Any of the green missing? I have never had a pest issue with pansies, but I suppose rabbits or deer might enjoy them...
Only thing is that these are in a wicker basket hanging on a fence. I can't see a rabbit getting up there. Would a squirrel eat them? Or an anole? We have lots of them around. Birds?
I would think deer if they were up off the ground on a fence if you have those around. The only bird I know who steals pansy leaves... rather just chews off the stem without eating anything is my tiel.