I don't really see any of the eeveelutions that bad compared to the rest (design wise). But, I think Eevee looks the best. If I had to put them in order of what design I like the best to my least favourite, it would go:
In terms of Desigh i like them all equally exept lefeaon which is the only one im pretty indifferent about. If i had to choose m fav would be flareon, but i wish he didnt suck as a pokemon. Would i really be too much to ask to just give the poor guy Flare Blitz XD.