They all have their flaws.
Flareon, leafeon and glaceon are more or less useless through typing and stat distributions (the eevees all have the same base stat level, just in different places and combined with their typing and movepool, these three suffer most).
Umbreon is my favourite pokemon but even my tinted specs can see it's useless at attacking anything (save for a neutral/SE foul play on frail opponents) and gets murdered by physically offensive mons carrying fighting moves (i.e. most of them) and knock off.
Jolteon and vaporeon perform their roles pretty solidly if unspectacularly.
My nomination for least favourite is sylveon. On battlespot it pretty much relies on cheap assed pixilate hyper voice spam and is usually used by cheap assed faggots with cheap assed dark void anything, cheap assed skill swap spinda/arcanine, cheap assed beat up whimsicott+flinch, or a cheap assed late game groudon to make up for their inability to do anything useful with non-OP mons.
Did I mention it's ability renders it cheap assed?
Also it looks fucking shit.
