espeon is the best: fast, special sweeper, good move, hp not bad, doesn't have a type that won't effect him
jolteon is 2nd: fast, you can buy t-bolt, a good amount of pokemon are water, isn't hit by lightning [hp/\]
vaporeon comes next: hp rocks, alot of enemies have rock and ground types, and there is the section at the end of 2 fire maps, not hit by water [hp/\]
umbreon: negates psychic, which is common for bosses, great defense, needs an offense ally to use helping hand on, isn't hit by psychic
flareon: great attack, but fire is special. weak against alot of types, but has power up ability of fire [fire/\]
stick with your favorite. its your choice