My favorite back in time was Vaporeon, all I needed was a water stone and voila; I had a fancy water type pokemon. In this new play through, I had a fun time evolving an eevee into an Umbreon for Nerdy in case she trades with me, so that would be my favorite now. >> I can't touch that one so I will have to train another one for me. Or I may go for an Espeon as well since I like Psychic Pokemon as well.
I don't really use Eeveelutions much, so I can't base them off power. But, if I were to order them based on looks, the order would be like this:
8: Vaporeon - I don't know why, but it never appealed to me that much
7: Jolteon - I like the spikes, but they seem overused to me
6: Leafeon - It looks like a tree with legs... Still cute though
5: Sylveon - I like the desgin, but the ribbons and bows really are too much!
4: Flareon - It's cute, but a little too fluffy for me...
3: Umbreon - I love it's dark colour and cuteness, but what's with the odd yellow rings all over its body?
2: Espeon - Simply because, it's unique, and looks the way a psychic type should!
1: Glaceon - Aw, come on... Do I really need a reason? Just look at it!
If I were to add Eevee itself to this list, it would go between Glaceon and Espeon
In terms of look, I'd say Leafeon. I really don't know what attracts me to this particular Eeveelution since nothing really strikes me as pretty or amazing (and as far as design goes, Leafeon doesn't boast anything I'm usually interested in).
In terms of usage (in battles and actual gameplay), I favor Jolteon.