I dunno, there are quite a few that I think are beautiful. This doesn't make them my favourites, or anything, but aesthetically pleasing anyway, and I realized when thinking about it that my standard for that seems to be smooth, swooping lines. Milotic, Suicune, Dewgong and Pidgeot all have those qualities. Chimecho too, in a way, but I think I'm influences by my imagining the sound of chimes.
FWIW, my faves are Snorlax, Swampert, Mamoswine, the Nido family, Electrode... I like looking at them too, but they don't say "beauty" to me. "Kickass" maybe, lol.