Fallout angelic cross, definitly ! (10 nuked levels sounds fine to me)
Maybe one or several pacifist cross, for killing as few enemies as possible
I'd be tempted by some "lucker" medal, like "felt 3 times or more under 5 hp". Btw, this one wouldn't be interesting in term of challenge, but could be a nice ornament to runs with real close situations.
Something for gibbing everything ? Wait... it's completely retarded.