ZSS is a tough one considering priorities and speed, not to mention many follow-ups with some of her attacks. D-Smash, B-air, and Side-B are those to watch for as mentioned before. Also, be wary of those who make good use of armor pieces, and can bait you to using reflectors. If you're good at managing and using items (which a good Pit should be good with items) as well as knowing what the ZSS would do, you'll be covered for the armor piece thing.
G&W is a well-known threat to Pit and should be watch for. As said before, avoid filling the bucket, though there's a way to use it against him, which I prefer to use at his last stock if possible. If you shoot an arrow, G&W has some bucket lag, allowing you to slowly approach him, and shoot another arrow (this should be the G&W's 2nd projectile the bucket has filled), and you can approach again and hopefully land a killing blow if he's at high %. It's fun to do on his last stock, or if you really need a kill. Don't fill the last part of the bucket though since it has much less lag.
You can also do this tactic on a recovering G&W (with empty bucket) who already used his midair jump to make him slowly fall and make him lose a stock if done correctly.
Now, if you do fill the bucket, there's a few things to know besides that strong KO potential it has against you:
1: G&W can't bucket break during this time.
2: You can start abusing arrows more at longer ranges, though some of G&W's move can still take out arrows.
GL though, since G&W is much more annoying.