I don't know.... I used to think that the Rocking Beats and Perfume Puffs were pretty ugly but then I saw Tuney's photo shoots of them. I like them a lot now, especially the Rocking Beats. So very 80's!
So I don't have any ponies that I think are ugly. I guess my least favorites now are the Talkin Ponies (Broke Back Pony!) and the Dance and Prance Ponies.