I
love to fight Peach. Samus sure feels like a Peach counter to me.
I'd say one of Samus's biggest advantages over Peach is how freaking easy it is to hit her with a charge blast. So many of Peach's moves
feel safe from a distance, but are wide open to charge blasts. If she's pulling a turnip and is half a stage away from you, you can hit her with the blast. If she's jumping at you and throws a turnip, you can hit her with the blast. If she's floating directly at you, shoot slightly below her (but still high enough to hit her). If she doesn't have a second jump, she has no chance of avoiding it. If she does have a second jump, she has a fraction of a chance. If she's dumb enough to do a dsmash and you're not with it it's range (like many Peaches get accustomed to doing), you can hit her with a real easy charge blast. And also, your typical set ups like hitting off of tech rolls and rolls still do wonders against Peach. So any time that you have time, charge a blast up, you'll probably have an opportunity to shoot it.
A lot of Peaches, as I've noticed, like to Shield grab a lot. If you see one trying to do so to you, do a dair, l cancel into a jab, and then dsmash. If she continues to shield the entire time, the dsmash will push you out of her grab range. If she tries to shield grab the dair, the jab and dsmash will hit her. Take advantage of the fact that when l-cancelled, Samus has the least amount of landing lag of any of the higher tier characters.
Fsmash ***** Peach. It outprioritizes all of Peach's aerials. If she's floating at you, a good strategy is to just walk away and then smash her when she's within your range. Ftilts also a good move. Good to put pressure on her shield, good for knocking her around, great range.
Recovery's a bit of a pain if you're coming in low. Turnips can screw up bomb jumps, but you can actually use them to your advantage, by letting the turnips hit your bombs while you're still near them, thus giving you the jump of the bomb without getting hit by the turnip. But that's more of an art, I don't know how to teach that
Without using your second jump, try to get underneath the ledge, then jump up and use your grapple to grab it if necessary. Ledge hopping on and cancelling into a dsmash seems to work wonders.
Your main kill moves on Peach should be Charge Blast, Fsmash, Ftilt, possibly uptilt, possibly sex kick, and missiles. There's a number of good ways to get these off on her, too, even against really good Peaches.
