To be honest, I like all of them; it's kind of tough not to, haha. Of them all, however, I would say one of my most favorites would be Vaporeon.
It's got an adorable-looking design, at least in my eyes; it's got features of both cat and fish mixed together, and they seemed to have executed the combination pretty well, frankly. If it were up to me, I definitely couldn't have imagined it to be any better. When it comes to battling, it's pretty useful, too; it's got ability to absorb water-type hits (regains health when struck by them), and has enough defensive stats to endure load of hits.
In future, I would like them to introduce a dragon-type Eeveelution, if possible. I could imagine this one to have dragon-like scales, as well as a set of small wings. It might be able to fly itself slightly from here to there, but likely won't have much of chance to carry its trainer around due to size of its wings and whatnot. When it comes to stats, I believe it would be wise if they were high physically (Attack and Defense).