Marder I 'Geschutzwagen', I find it to be quite an ugly Tank Destroyer due to it's bulky looks, but i'll leave you guys to your own opinions on it.
I don't have favorites when in relation to "beautiful" tanks, but in a list:
Churchill, due to how it's design was made for it's actual purpose.
M18 'Hellcat'
M24 'Chaffee'
M36 'Jackson'
Pz.Kmpf V Ausf. G 'Panther'
and the KV-1 and T-34 tanks.
oh and before I forget, I find the French B1-bis to be quite an ugly tank aswell, it just looks a bit like the TOG.