Who is the best eevee?

[Caterpie]

Who is the best eevee?

[Zidarose]

This is a very subjective question, but the most commonly accepted as the best Eevee forms would be Jolteon and Vaporeon, as ranked by Smogon.

Aside from that, most of the forms have numerous benefits and you should look into The Evolution of Eevee, it's a good guide written to help people choose the form they're looking for in terms of need.