Worst Eevee evolution?

Leafeon. I don't like its name, I don't like its design, and I don't like its stats. Case closed.

This is a tough one for me since I'm not much of an Eevee trainer, and wanted to stay neutral. But I chose Leafeon because I have never been fond of its design and IMO, it's the one that least resembles Eevee or an Eeeveelution.

Every eeveelution is great, but my least favorite is Glaceon. I don't care how well they perform in battle. Glaceon's still cute, but out of all of them she has the least creative design. She's got long ears, some sort of helmet/hairdo sort of thing on her head and a diamond on her back. WOW. That's probably because she came out of Gen 4 though. IMO they could have come up with many better designs for an Ice-Type eeveelution.

Glaceon, by a significant margin as well. I'm not a big fan of pure Ice-types from a competitive standpoint due to their near complete lack of resistances. That, coupled with a fugly design and boring-ass moves means that Glaceon is at the bottom of any Eeveelution list compiled by me. Apart from 'Worst Eeeveelution', which it would win hands down! :D

The worst (since that is, after all, the question the OP asked) is Flareon, absolutely. Sorry, but that's just the way it is. The only thing it really has going for it is good Attack, but it doesn't have any decent moves to take advantage of that, and even with better moves it'd still be too slow to be useful.

Personally though, my least favorite (an entirely different question) is Glaceon. It's okay as Ice types go, but the design just doesn't appeal to me as much as the others.