To be honest, Eevee's evolutions are all pretty good, except for the fire elf, which is not very useful. Everything else is good. Since you already have Emperor Penguin, just give up on the water elf. Thunder elves and sun elves have attributes that you don’t have among the 4 currently. These two are good for strong attacks. Dark elves generally use more tactics in regular battles. At least I don’t use them much when clearing levels. Ice and grass are also good. Yes, what I personally find more depressing is why the grass elves have high general attack but not high special attack, and the ice type is slower and has more weaknesses. . .

Speaking of being powerful, I cannot say that this one is particularly powerful. As long as the tactics are used properly, no matter which one of the seven will make a difference, it basically depends on personal preference. I think you could try thunder or light.