blazeblitz: I just guess they couldn't be bothered with another Eeveelution when they made Ruby and Sapphire.
Ive decided Vappy is the most useful on my list, with good Sp. Attack and HP, speed is average and so on. Jolty is OK. Its speed is uber and so is its Sp. Attack, but HP and (Sp.)Defence are low so it gets wiped out a bit...
Espy has great Speed and Sp. Attack making it totally pwn the opponent if it's fast and strong enough, but it has appauling (Sp.)Defence and Attack. Good for getting rid of the enemies on a first strike, but if they're strong enough to survive, Espy is in a bit of trouble.
Umby has bad Speed and Sp. Attack, average Defence and Attack and great Sp. Defence and HP. It's not a very good attacker, but a great defender. I think out of Espy and Umby, Espy is more powerful, but Umby does have the power to survive for a long time, I'll give it that.
Flareon, I've never used properly, but I've heard that it has good Attack and Sp. Attack, so if that's the case, I think it should be quite strong. Not as strong as Espeon and Vaporeon though.
So, with srengths, I think...
Vaporeon/Espeon
Flareon
Jolteon
Umbreon
That's only my opinion though.