Okay this is two questions cause I’m fighting between four servents. Cu Alter, Minamoto for there respective class, and Merlin and Zhuge. If it helps I run Altria saber, and Gilgamesh archer.
Cu Alter for ST buster survivalist. You already have enough farming potential in Arturia and Gilgamesh. That said I use my Raikou far more than Cu Alter, because she can be taken to any battle (besides stuff strong to zerks, which isn't much) and boss kill, wave clear, not as strong in drawn out battles though.
And Memelin and Waver will always be two you should aim for if it's for gameplay.
Waver or Merlin, both is totally broken. No matter what you choose, both can provide you easy mode of gameplay.
Merlin/Waver can supp almost anything.
Raikou need Waver/Merlin or another supp.
Chu Alter is leaning more into independent unit and survivability.
Since you're using Buster Crit servant like archer Gilgamesh and buster saber like Artoria, Merlin is your best choice and he can turn every Buster servants in your rosters into a monster.
Waver carried me from early singularities until I got Merlin. Both are stupidly good supports and using both at the same time should enable any servant to do well. Heck, using either one without the other will likely enable any other two servants you use.
Cu Alter and Raiko are great servants, but either of the two casters can be used to make any other servant in the game significantly better, including those two if you should happen to get them at a later date.
Cu Alter because Gil covers you for AOE. Merlin because both your mains are buster. Waver's good too though.
I would go with Merlin because OG Artoria and Gil are buster servants. Any buster servant appreciates Merlin's buffs so I would go with him. If you had a more diverse deck type (like mix of Arts/Buster/Quick) then Waver. Overall, you can't go wrong with either Merlin or Waver. Getting either/both will make it easy for you
