I'm late, but I had an opportunity to play against ZSS in tournament again a few days ago, and I'd say the matchup isn't as bad as -3. To get out of juggles/D-throw setups, all you have to do is DI as far away as you can; D-throw to up-B should never be able to connect at a % where it will actually kill Charizard, and even if it does hit, you can either DI out of it during the multi-hit, or the trajectory on the final hit is easy enough to DI such that you'll often be able to survive that hit up to around 120% (140% if you're hit in the wrong direction). Also if you DI way out, you can quite safely Flare Blitz back towards the stage, since the move is actually really hard for ZSS to challenge in the air (and if she tries, you'll win every trade). ZSS' Paralyzer is largely unsafe against Charizard, who can Flamethrower or Flare Blitz right through the shots. When using Flamethrower, you have to stop the move before it stops hitting so you can shield the buffered Flip Kick that will likely be coming (you may be able to get a punish if you powershield it, maybe with a hard Fly read if you're up to it). On that note, Flip Kick is the move that really makes this matchup difficult, since it's hard to punish and is able to kill you much earlier than any of ZSS' other moves; on certain stages, you're going to need to be extremely patient if ZSS starts camping and fishing for a Flip Kick. Something important to know for this matchup, though: Charizard's F-smash can hit ZSS on the ledge. I'd say the matchup is a -2 at worst, if the opponent camps (try to avoid Delfino and Duck Hunt if you can), but it's much closer to even if the ZSS has to approach; you can snowball hard if you get a stock lead.
Captain Falcon can be a difficult matchup, because he hits almost as hard as you do and takes hits almost as well as you do, except he's faster in every way. Captain Falcon also barely has to commit to any one action, making it hard to mix him up for a safe landing out of a juggle (but unlike ZSS, Falcon's juggles actually hurt); landing with Rock Smash is incredibly unsafe here. What you have over Falcon is range on normals (watch out for F-smashes though), and with no projectile, he can't camp you out. Charizard can actually play the defensive role in this matchup. Captain Falcon can't really rush in on your N-air, and Fly is a really strong out-of-shield punish option. If you get Falcon off-stage, you can quite safely Flamethrower the ledge while he's trying to recover and rack on some extra damage for free, since the fire will repeatedly hit him out of his up-B (I'm pretty sure it even beats the custom up-B that turns the move into a hit instead of a grab). Wall him out with jabs, Flamethrower and disjointed tail/wing attacks. Safe defensive play isn't usually Charizard's style, but for a hard-hitting rushdown character like Captain Falcon, that's probably what you'll have to resort to. Either 0 or -1 for Charizard; you're slower, and you can be punished hard, but you do have a range advantage.
Donkey Kong is an interesting one. Both Donkey Kong and Charizard excel on stages with platforms, and have really good moves for keeping opponents off the ground. At first, it may seem like the matchup is in DK's favour, since Charizard's aerial mobility is awful, DK hits hard, is heavier and has longer range on most moves, and Kong Cyclone is a way better up-B than Fly. But there are two important things that make the matchup a lot easier for Charizard: DK gets shield-poked very easily, and you can fight Kong Cyclone. Rock Smash will shield-poke DK 90% of the time, making it really safe landing option. Flamethrower will let you pressure a shield, approach Hand Slaps or cover the Kong Cyclone option by stopping it during startup. Since Kong Cyclone relies on lagless landings on platforms, and Charizard's U-smash and U-tilt cover platforms extremely well, you can actually pressure DK when he uses it. Rock Smash or Flare Blitz vs Kong Cyclone is some stupid stuff. What if they're not using Kong Cyclone? Free D-air spikes, this isn't even a fight. This match up is probably +1 (+2 with no Cyclone) for Charizard. You don't win trades, survive as long or kill as early as you would in most matchups, but DK's awful shield and massive hurtbox make this a lot easier than it really should be.
Also, for stages, I can't think of any matchups where you wouldn't want to go straight to Battlefield, not even Kong Cyclone. Other stages with platforms have their merits, but always go Battlefield when you can. Otherwise, Smashville, T&C and Halberd are usually stages you don't mind. (Slopes on sides of Halberd and Lylat can mess up your shorthop game, so be careful with those.) The other stages tend to allow too much room for some characters to camp you out.