Ooh, Peach discussion? Count me in. I've played a lot with Hack in the past couple of weeks and I'm starting to get a really good grip on the matchup.
@Eryx
Yeah, turnips can be a pain to deal with. Powershielding is indeed a valid option. Still, the best way to deal with them is to not let her pick them up uncontested in the first place.
Gonna ramble a bit:
I view the Peach matchup a bit like playing golf in the sense that you can never afford to lose your patience. Frustration will spell the end of your stocks. Don't throw out predictable fsmashes in the neutral game just because you're tired of her living to 180%. Don't start approaching mindlessly with nairs because you've ran out of ideas. Being at a stock disadvantage against a character that excels at trading and survivability is frustrating, but you'll simply have to suck it up and stay focused if you want to have any hopes of turning the match around.
Now, "patience" doesn't mean "no aggression whatsoever". It just means that regardless of whether you're on the offense or defense, you'll be punished badly if you try to force openings that aren't there. Be systematic.
Okay, moving on to more specific stuff. Just like pretty much any other matchup for Marth, you don't want to be the one who jumps first. In this particular matchup, the main reason for that is Peach's dash attack. That annoying-ass move is fast, has more range than it looks like, and has a habit of sneaking under Marth's aerials most of the time. Not to mention the fact that it's a good combo starter, especially against an airborne opponent.
Aside from dash attack, there's also the fact that a grounded Peach at not-high % can CC punish pretty much any aerial that isn't spaced and timed perfectly.
So before you can start using aerials, you generally want to force her into the air. This is mainly done through dtilts and grabs. Dtilts because they have good range while at the same time being fast and very difficult to punish, and grabs because they circumvent Peach's shield, which is extremely good at avoiding pokes.
If you land a dtilt (or several) but Peach decides to stay grounded, just be grateful for the added damage and stage control and keep doing what you're doing. Don't get impatient.
If you land a grab, upthrow should be your go-to option. Yes, fthrow and dthrow can occasionally reward you with some pretty good follow-ups, but most of the time you won't get anything out of them (this is particularly true for fthrow), or at least not anything nearly as good as what you could've gotten if you learned how to juggle properly.
This is not to say that your other throws don't have their areas of usage, but rather that they're not as consistently good in general.
Might add more later. Getting Peach into the air on your conditions is, in my opinion, the first major hurdle to pass and the key to this matchup, so I wanted to focus on that first.
Also, while most of the advice in this post is targeted towards newer players and/or players that struggle with this matchup, I think the bit about patience is a point worth reiterating for everyone.