Musashi or Gramps?

They are both good, you can't go wrong with either. Both go together well with merlin if you have him.

Musashi makes better use of what Merlin has to offer to deal very high one-turn damage, while King Hassan is slightly more independent than Musashi, and has better sustain but deals less damage.

Since Okita is also pretty close, and she's really good too, imo you could go with king Hassan for now and go for Okita later. King Hassan and Musashi both get another rate up later in the year too so you have another shot at both of them anyway.