Who to roll for

Well the best answer to that question is usually Zhuge Liang and iirc he is featured in a summon banner sometime soon

I second this^ and after him merlin, you can't go wrong with either of them, since you will often need them for farming or hard content, once you have your own you will have a lot more temp comps available and farming will be a lot easier

Zhuge Liang and Merlin are really the only servants who fit this criteria, with Merlin being a bit debatable outside of buster comps.

For general use as mentioned Zhuge Liang is the best all around support, followed closely behind by Merlin. Also a strong Berserker like Cu alter or Moma Raikou or Herc will carry you very far. Suggestion though: read up on the Servants, and find one you like. Try and test them out via supports or story missions, and then save for that Servant. You will get more enjoyment out of playing a Servant you like than one high up on the tier list. While it IS good to have Merlin or Zhuge, they are also very easy to get from the support list. I played almost through the entire game with them and still had a ton of fun.