Usmash is our worst move, but still has it's uses. Can cover platforms from below and such.
Nair is good in places for sure... knocks ICs apart so you can go after Nana, in addition to what's been said. Also usually a safe move for zoning since it has almost no frames of lag in either direction.
I like to use AR to punish spotdodges, just remember to stop hitting B before they escape. Last few hits + windpush make you relatively safe if you time it. Also useful for reflecting stuff like Thunder or Snake's Usmash that comes at you vertically.
Never use Usmash to hit people using dairs like Sonic's or Sheik's, they can hit through it if the timing is off. Uair is a thousand times better for this, Sonic can't do crap against it. Uair cancels out his bumper too. Utilt is still better if you don't have time to jump, never had it fail on me.
Utilt is a fantastic move to end a spotdodge war, if you're both on top of each other. Also good out of autocanceled Nair, since it starts up on frame 2. If they spotdodge your Nair, why not? Good damage too. Just awful, depressing range.
Jab is great for punishing without dedicating yourself to anything, and stopping some approaches.
Fsmash is wonderful for punishing (Frame 6, 19% I think?), and good for kills if it's fresh or you're having a hard time landing Bair
Only use Dsmash to hit in front of you, and be careful when you use it. If you miss, you'll get punished as Pit clumsily turns around to hit behind him. Even if the back hit gets them, it's nothing special. I hate that back hit. Don't use it as a GTFO move like some Dsmashes (Sheik, Zelda, etc.)
Dtilt is good range, pops people up to be juggled, and is faster than Ftilt for it's range.
Ftilt is probably the best ranged ground move, comes out slow but kills fairly well and at a low angle (Unlike Fsmash, which sends them up a bit). Safe on shield a lot of the time.
Fair is good for spacing, gimp kills, and just knocking people away. Little slower than I'd like it to be, but range and power make up for it.
I never use dair, probably should.
Bair is our best kill move, annoyingly hard to land though. Good for gimping people with poor recoveries because it stays out so long. Sometimes score stage spikes with it, too.
Uair is a sexy sexy beast. Good damage, hard to cut through, great for sharking, great for ledgecamping, great for almost anything above you (unless it's something like Link or Ike's Dair). If I was forced to play Pit with only one move, I'd choose this one. I've had it kill, too, though they have to be fairly high. God this move is so easy to land. I love this move.
Down-b is self explanatory. Reflects things, capes people around. Kill space animal Up-bs, Ness and lucas up-bs, Ike's up-b, Pika's side-b (or just arrow him), just about anything. Also, some of the laggier dash attacks can be reflected on the ground. Can also be used to shield hits in the air, but it's not as useful for that as I'd like it to be. First few frames has super armor.
Up-b... lives up to it's name. Seems great at first, gives you great mobility and great recovery, but fly to close to the sun (or get hit at any point in the move other than the very start) and you lose all your jumps, your up-b, everything. Try not to use it unless you have to. Wing refresh is awesome against some characters.
Neutral-b is what makes Pit, Pit. PewPewPew. Forces approaches, punishes everything done far away from you, gimps people off the side, loops for fancy taunts (lol) charges to change up the timing, generally fast and not too much lag afterwards. Everything you could ever want from a projectile.
Oh, and my advice may not be the best, I'm not hte best pit around, but that's basically my opinion on our whole moveset.
