>>530127523
The only two "must rolls" are Oberon and Castoria, after them roll for who you like. Skadi helps if the servants you like are quick, same with Vitch and buster.

Frankly, you don't even really need to roll for the must rolls, they're just an incredible QoL improvement. The game definitely isn't so hard that you need them to progress in the story, they just come very highly recommended since they smooth everything out.