Diddy dunks on ZSS and many say he beats Sheik as well. He doesn't do you any favors against swordsmen though, and Cloud can beat most of them with his superior range and mobility, but leaves you open to Sheik. Sonic can handle projectile characters most of the time by going in before they can set up camp.
The crewmate I was talking about plays Megaman as a secondary, who can zone out most swordsmen, and beats ZSS in neutral, though idk how he does vs Sheik. You might try that out and see how it goes. Megaman also has the advantage of relatively few people knowing the MU, so that can work in your favor.
- Covers ZSS and Sheik, loses to swords
- Covers swords and ZSS, loses to Sheik
Theoretically covers everything except possibly Sheik? pretty well, gives you the advantage in matchup knowledge
Overall I would probably suggest Megaman, but I'm only one person.