Which guaranteed banner?

Nice job on grabbing Musashi, it looks like you've got some phenomenal buster dps for the knights so I'd skip those altogether. I'd highly recommend either the Caster banner for Waver for his stall potential, damage/defense/critical up. Or Tamamo who could really save your bacon with helping NP gain and skill cooldown. Sanzang isn't the best to get but she also isn't the worst, shes a great addition to a Buster team and has a decent ST Buster NP. The only other I'd recommend is the Ruler/Assassin banner. 50/50 on either getting an amazing crit star generator and ST Anti-Female Assassin (Jack), or (at least in my opinion) the best Ruler class servant in the game whose only fault is her NP which is getting an interlude in May. Good luck with your rolls!

Personally I'd recommend either Rider or Caster, but more on the latter.

Rider has Quetzacoatl and Ozymandias, both great Servants. Drake is also a good choice, though mostly for farming from what I've seen of her. Medb is pretty 'meh' though. While she does get upgrades to her skills later on, I still think she falls short of Ozy and Quetz. That said, there are four of them so there's only a 25% chance of getting a specific Servant. If you want only Quetz or Ozy, that's still a 50% chance of success and a 50% chance of disappointment.

If you go Caster, you have a choice between Zhuge Liang(Waver) who is an excellent all-around support regardless of which of your Servants he is teamed up with. Tamamo is an excellent Arts support Servant and would go very well with Archer Arturia and Berserker Vlad. Sanzang is an offense-oriented Caster with a bit of team support. And while a Berserker could easily deal more damage than her, she does have the advantage of not receiving double damage from every class and she has Damage Cut (up to 1500 damage per hit) paired with her Taunt.

Either go with Rider and use a friend's Waver or Merlin for support, or you could go Caster and borrow either a second support or a powerful DPS to support. Either works.