The A330 and A340 flatter any livery that is applied. Think of a livery that looks ugly on these planes. Bet you can't.
Without wishing to be too much in the Airbus corner (although I am, they make fantastic state-of-the-art aircraft), I think the A320 family (esp 320 and 321) make most liveries look boxy and unsexy.
fly Saha Air 707s daily from Tehran's downtown Mehrabad to Mashhad, Kish Island and Ahwaz