At low percents, dthrow (only red/yellow/blue/white) will combo into fair or usmash. Or Nair, I suppose. If you follow up the dthrow with a fair, you can sometimes land a regrab on some opponents, leading to another fair or usmash from the dthrow. After an usmash, you can land an uair sometimes, which is more reliable. On top of that, you're still grounded, which is better for juggling after the usmash, if you don't go for the uthrow. So I think dthrow -> usmash is the over all best option. If the opponent has some percents on them, but still enough to combo, dthrow -> purple pikmin toss -> fair is legit, but I don't know the exact situations/percents that it works.
White usmash at low percents (about 8% and lower) combos into about anything you want. Yellow does to but only if it was the move you killed your opponent with last stock and it's the first move you hit them with on their next stock.
Dthrow -> dthrow is only a combo on Fox and Wolf. It's a legitimate string on some characters (Snakes often won't jump after a dthrow, so it'll often work on them), but it isn't a combo on anyone else.
Purple dthrow does not combo into anything. Don't try, lol. Neither does uthrow.
Spinny thing to up smash combo.
Uair if the last hit doesn't connect will sometimes string into nair. You have to push above the opponent and they'll have enough hitstun to where it'll be too difficult for them to get away. Dthrow -> Up Smash -> Incomplete Up Air -> Incomplete Nair -> Up Smash -> Up Air is a favorite of mine. Not going to put a whole lot of emphasis on this in the guide, though.
After a purple pikmin toss, landing a grab afterwards is incredibly easy, the way it knocks the opponent off balance.
Other people can talk now.
Edit: Going to list what characters you shouldn't try to use either fair or usmash on out of a dthrow. Most of these will be because of either weight, fast aerials/specials, or a combination of the two. Anyone not mentioned you can do usmash or fair on, but anything listed is, imo, what you should definitely do on these characters.
Falco: Fair can be difficult to land if the Falco DIs right. Which doesn't matter, because landing the uair after an usmash is easier than it is on most characters, because of how fast he falls. Go for usmash on Falco.
Marth: Marth can often Dolphin Slash if he predicts you're going to usmash. He can't do this if he's at really low percents, but if he's at, say, 10% (which is still usually combo percents), if you aren't spot on in the dthrow -> usmash, he can oftentimes break the combo. Dthrow -> Fair is recommended.
GW: His Floatiness makes dthrow -> usmash a tad difficult to land, but nothing impossible. Go for it if you'd like, but Dthrow -> Fair is a lot more guaranteed.
Lucario: Do not try to Dthrow -> Usmash him! A combination of his floatiness and frame 4 dair that completely out prioritizes our usmash makes it almost never land. Dthrow -> Fair, on the other hand, is extremely easy to land.
Toon Link: I prefer Fair after a dthrow because there's no chance of his Nair getting in the way, lol. However usmash isn't extremely hard to land, in all honesty.
Kirby: Use Fair. Floatiness reason.
Fox: Dthrow -> Dthrow -> Dthrow -> ____. One of the few characters that dthrow is a legit combo on. Your second and third dthrows can't be reds, though, or it won't work. The combo ends with those if they are.
Rob: I prefer Usmash because of the horrible situation it puts rob in (above us).
Peach: Fair because of her floatiness and dair which puts a halt to usmash.
Luigi: Same as above but replace dair with nair. Use fair against luigi and space it perfectly. Don't try a followup after the fair.
Wolf: Same as fox.
Sheik: A character I think dthrow -> dthrow is a legit combo on, but possibly only with certain colors. Going to do more testing. Also, I prefer fair against Sheik, putting her above us isn't a huge benefit.
Sonic: Fair if he's not at extremely low percents. If he has a few percents on him he can spring away.
Ike: Same as above but replace spring with... Nair <__< followed by jab followed by more jabs.
Yoshi: Fair. Exact same reasons as Luigi.