you seem to overuse Shiek then Zelda too much, when just Zelda would be so much better (also because changing safely is very situational)
Bowser is definately Zelda all they way, infact any char thats big and slow (and sitting ducks for the fair/bair, and sometimes dair) should be just Zelda This includes, Bowser, Ganon, and Donkey Kong (he gets ripped apart by Zelda's Dair). The exception may be Dedede, Sheik seems to get through his spam easier than Zelda, yet he's so big, he's almost as easy as bowser to kill with kicks, so i'll leave that as undecided.
Though Sheik then Zelda IS a good plan, since being able to rack damage up fast and kill with the same character seems godlike, the thing that balances it out is the transform time There are very few ways to do this safely and it is all situational. Because of this, even though Sheik to Zelda is an awesome choice, its probably much safer to just choose one since you don't know if you'll be able to change or not. When I need to choose only one char, I usually choose Zelda, since she can rack up damage and kill (with the exception of matches that are usually horrible for Zelda, like Lucas)
@imdavid
no Sheik generally fares better against Pit than Zelda, any decent pit will jump and shoot, or bend arrows, so that makes NL worse than a shield because it won't reflect back at him. However, its not the end of the world, I've found the arrows are negated almost completely with advancing powershields (takes practice but VERY useful against campers), though this forces you to approach, and sheik generally has better approaches, so I would say Sheik has a slight upper hand on this against Zelda and thus you should use Sheik against Pit
Back @popsofctown
let me go over the changes I would make to your list, and I'll add what char you should use when only using one char, when you think you can't pull off StZ on an opponent: (I won't list people I'm not changing)
Bowser: Zelda definitely
Cfalcon: Zelda (the reason I'll put Zelda to any char with weak/no projectiles is because they are forced to apporach Zelda, who has one of the best defence games in SBB)
Diddy Kong: Zelda
Donkey Kong: Zelda
Falco: Sheik, she is definitely better at killing space animals then Zelda
Fox: Sheik
Ganon: Zelda
Ike: Zelda
King Dedede: undecided (both have their strengths and weaknesses), probably StZ is good
Link: StZ, or Zelda (Shiek and Zelda are both equally ish good against Link, yet this is a character with a horrible approach who must rely on spam, and Zelda's spam is better, forcing him to approach, giving Zelda a slight edge over Sheik)
Luigi: StZ, or Zelda
Mario: StZ, or Zelda
Marth: Zelda, (lacks projectiles, and also an unusually large target for fair/bair for his light weight)
MetaKnight: StZ, or ???? (can't yet say who has the upper hand against MK)
Peach: StZ, or Zelda
Pit: Sheik (I've explained above)
Samus: StZ, or Zelda? (not sure who has the upper hand in this one)
Toon Link:StZ, or Sheik?
Wario: StZ, or Zelda (fat, no projectile, slow, done)
thats all I have so far, let me lurk some moar for info on all the ?s