Pick a servant from each main class as who you would want in a grail war, but you have the following rules, No more then one 5 star, no more then 4, 4 stars, No modified forms(Ex Santa or Summer forms, Alters, (Also Consider your location effecting things ex wether vlad is a zerker or Lancer ), You have to own them in-game.
After that Pick the one you want, knowing the rest are your opponents.
My picks:
Saber: Nero Claudius.
Archer: Atalanta
Lancer: Nehza
Rider: Francis Drake
Caster: Avicebron.
Assassin: Corday
Berserker: Fran
Who I would pick?
Nero or Atalanta, because I have the best odds of winning with one of them I believe, of the two I think I would pick Atalanta because her Wish for the grail is really wholesome and would be so nice to see come true. I think Atalanta has good odds to win because she could manage to hold her own and get a mutual kill on Achilles, and even with him being weakened from my understanding it is still quite the feat. I wish I could make all their dreams come true though and I wouldn't mind any of the Servants I choose winning.