Doc has easily one of the best projectiles in the game, so use it a lot. He can chaingrab a lot of people with his dthrow, and fast fallers like fox, falco and falcon he can chaingrab with his uthrow at lower percents, then his dthrow at higher percents. As for his moves, here's how it breaks down.
SMASHES
U-smash: IMO his worst smash. Don't get me wrong, it's still good, but his fsmash and dsmash are better. In a lot of situations his utilt would be better
F-Smash: Great damage/knockback but poor range. Use it if you know you're going to get a hit and punish your opponents lag with it.
D-Smash: Good range, great knockback. IMO his best smash. Great after some shffl'd uairs
TILTS
U-tilt: IMO his best tilt. If done while close enough to your enemy or directly under your enemy, it will put them directly above you where you can proceed to uair shffl the hell out of them
F-tilt: Doc's farthest reaching move (correct me if I'm wrong), and quite quick
D-tilt: His worst tilt. It's a fact. I can't give you much info about it since I never use it.
AIRS
U-air: All of your combos will most likely involve some shffl'd uairs here or there. It's my favorite juggling move in the game as you can move across the stage very fast while shffling it. Also, you don't need to L-cancel it if you do it right after you jump (so really, it should just be shff'd uair but people still say shffl'd
)
F-air: One of doc's best killing blow moves. Uthrow or Dthrow your opponent (depending on how heavy they are/how fast they fall) and finish with a fair for the kill. Punish your opponents mistakes with a shffl'd fair.
D-air: I probably don't use this skill as much as I should, but it is his worst air. Again like the usmash, don't get me wrong, it's not bad, but it's just the worst out of the five
B-air: Arguably doc's best aerial. Amazing edge guarding move and great if you waveland it (Called a SHBAWD)
N-air: This is a good sex kick that does exactly the opposite of other sex kicks, and that is, it gets stronger the longer it's out. Put a few of these in your approach.
B MOVES
Neuteral B: As I said, one of the best projectiles in the game. Spam it but don't let it's lag get you killed
Over B: The cape. Quite a versatile move, yes, a versatile move indeed. You can use it to reflect projectiles back at your enemy at double damage, it will help you recover, it turns people around (good for edgeguarding people like ganon or falcon), and you can easily turn yourself around in the air and grab the ledge (hard to explain... watch SmashMac's video).
Down B: The tornado really should never be used in combat. If you've mastered the PTP (meaning you can go as high as possible with this move) use it in your recovery before your doublejump.
Up B: Aside from being a recovery move, this moves only other usage would be the Up B cancel, which is very hard to do, but can be effective.
I hope this helped you. Go watch SmashMac's video and you can learn how to utilize these moves properly.