It is odd of me to say that the back sprites of these Pokémon is what makes me like some of them even more than before? I've always loved the Treecko evolutionary line, but Grovyle and Sceptile's back sprites are really well done. I didn't think it was possible for me to like them even more.
Swampert's back sprite is pure win though!
I've been a fan of Mijumaru since the start. The sprites for his evolutions is what makes me like them even more - especially with actual confirmation and a change from the leaked artwork. But, the second stage sprite is both cute and cool. Still wishes it's main body was white, like Mijumaru, but I can live with it.