stevebydac beat me to it with the wolf eel. charming fish, at least mine is so far. I love fish with character.

Other "ugly", but not to me, fish I keep:
wartskin anglers (plus a few others) - best in a species tank but do ok with corals
Hawaiian leaf fish - can't keep with aggressive feeders and they will eat all your shrimp
zebra moray - mine looks like Mr. Magoo in the face. gets big, can rearrange rockwork, eats inverts, never bothers fish.
orange toadfish - another ambush predator not safe with inverts or small fish
some people might think seahorses are ugly, I keep them too.
guess beauty is in the eye of the beholder.