What would people say is a good secondary to combat peach? I hear mixed stories about Marth, some people say he has a hard time while others say he's a solid choice, the only other options I can think of are Sheik and Puff, are either a good choice?
The Jigglypuff vs Peach matchup is an aerial battle that hinges on spacing things right. It's pretty even.
That being said, if you're considering a floaty like Puff, why not consider Peach?
She's a fun character to use, and then you know the matchup is completely even. And if she's your secondary, you can use her in other matches too to mess people up, because not too many people play a good Peach.
If you're looking for a way to just dominate Peach, however, I think Sheik or Falcon would work. Falcon beats Peach's aerials a lot and mainly grabs her for combos/kills, and Sheik has those needles, throws, and aerials to shut down Peach as well. Not that a good Peach player can't adapt to those things, but it definitely limits Peach a little bit.