Okay, I'm gonna share my opinion on the ZSS matchup, based on my (distinctly brief) experience~
ZSS's gameplan pretty much revolves around her grab. Be wary of it. If you spotdodge or roll towards it, you will probably get caught, since the node on the whip can grab you even if it's extending or retreating. Best best is to just evade it whenever possible. It's pretty easy to read and ZSS is wide open when it extends, so if it misses, you've got a free punish. DON'T GO FOR THE FRONT - Pit's disjoints won't beat the whip at maximum extension. Go for a shorthop d-air or b-air.
If ZSS tries to use her Paralyzer, don't challenge it. Remember that she can go for a grab if it connects or punish a hasty jump with an aerial. The move is psychological as much as it is physical: with the option to put you in disadvantage at her own discretion, you need to play very safe. Only reflect it with Orbies if you're confident she can't punish you for it. Don't try to Upperdash it, it will just open you up for the grab. Be wary of shielding it, because that just open you up for a grab. Depending on how she follows up from Paralyzer, spotdodge, roll behind or shorthop, then punish. Rolling back just makes it easier for the grab to connect. She may deliberately do this to make you move and then punish you for it, so study her reactions in detail.
ZSS's advantage state is disgusting. D-throw opens up just about everything for her, and if you get popped into the air, u-air, f-air and b-air will make it difficult to reset. Be wary of airdodges: ZSS will read them and punish with a u-air (which has a big hitbox, perfect for catching spotdodges and wonky DI). Instead, try to fastfall dodge to beat the punish. DON'T TRY TO CHALLENGE HER IN THIS STATE - top priority is to reset, don't make it easy for her to whip you about!
ZSS can reset from disadvantage pretty handy. N-air is a quick disjoint and, in a pinch, d-air can take her back to neutral in a jiffy. Don't get hungry for the followup: f-air will trade with her f-air if she goes for it, but remember that she's quicker than us for the most part and she's got an emergency disjoint in side-b and z-air if disjoints need using.
Z-air is a disgusting spacing tool, making it difficult to get close to her. I need to stress this: DON'T CHALLENGE HER DIRECTLY. Bait and punish. Never try to force her into disadvantage. Play it safe and make good of every chance you can get to catch her, but remember that she has three aerial disjoints (n-air, side-b, z-air) and two stun options (d-smash, neutral-b): if you make a mistake and get punished for it, expect pain. Don't forget that up-b is an effective punish / OOS option and can end the stock at high percents!
Off-stage, ZSS's aerials make it difficult to gimp her effectively; this, combined with open-ended recovery options (up-b, down-b, tether) makes it so you have to be ready for several contingencies. If we recovery poorly, down-b can end the stock with a spike, so be wary. Don't try to challenge her off-stage if you don't think you can trade with her, because she can just punish you and recover with ease.
I think arrows need to be used sparingly in the matchup. ZSS is so fast and versatile and has such good disjoints that she can punish us for trying to snipe her, so try to go for disjoints instead and keep it fast and safe.
Upperdash can be useful as a hail mary, same as every other matchup, but be careful of when you use it: grab will beat it, so wait for a good opening (for example, if ZSS d-airs onto the stage or you can evade her down-b) before you go for the kill.
I think this is definitely high up there in terms of Pit's difficult matchups, maybe even his most difficult. I'd say it's a solid 40:60 or -2.