hello! my gosh I'm sorry for the super late reply, I've been pretty inactive on here since June since some stuff was going on irl ;w; my apologies, but now I'm back!
hmm, I think it is all preference! they all have different strengths and weaknesses. I'm not really much of a competitive player though ... I use all the eeveelutions since I love them all! but if I were to pick, I'd say that I think Vaporeon is the strongest because of high HP, pretty good defense and nice special attack