Doc's down throw isn't B when he gets equal or more reward off of it than Mario. Toon Link's down throw isn't B when he gets literally nothing off of it and it doesn't kill. I also don't think Lucas's is better than Ness's, Zeldas, or Luigi's. Diddy's Dthrow isn't that good either and is strictly outclassed by Uthrow.
B -> S
B -> D
S -> A
A -> C
I have a hard time seeing Doc's as better than Mario's, since I believe that Mario's has more consistent followups. Doc's strings do more damage, though, and he can get kills with DThrow to FAir. I could see it being S.
Lucas's DThrow is definitely better than Ness's lol. Luigi is debatable, Zelda is mainly only there because of DThrow to UAir but that alone is scary. I'd still say it's S, as is has reliable damage, frame traps, and a kill confirm.
Ok so here is the preliminary list for Down-throw:
Discuss!
Diddy's should drop as mentioned. I could see B or C.
I could see Game & Watch going to S but it's your call. He can get a lot of damage from it and a kill confirm with a narrow window.
Link's could be A, he gets a lot of mileage out of his DThrow on most of the cast and occasionally has a kill confirm from it.
Bowser Jr.'s is bad and only has slight killing potential though I doubt it would kill earlier than UThrow or BThrow save for rare cases. C or D tier.
Toon Link's is bad. D tier.
You could make a case for Palutena in A; she gets DThrow to FAir guaranteed on most of the cast for a long time, and bad DI/option select means she can kill you with UAir, USmash, or BAir (the last might even be confirmed regardless on occasion).
Samus, the Pits, Greninja, Dedede, and Mega Man all deserve to move up. Possibly Mii Swordfighter and Duck Hunt as well.
Mac and Falco need to move up (possibly the Falchions too). Falco has low percent combos from his as does Mac. Mac may also have Rising Uppercut as a kill confirm from DThrow too.
Sonic, Mewtwo, Wario, and Roy either don't have anything confirmed from their DThrows or very little, but they make the opponent chose an option that can usually be covered or followed up.
Here's a little cheat sheet I made, may be a little off:
Might be wrong on some of these, especially the damage ones.
Finally ROB should certainly be S tier. He has one of the best, if not THE best hooh-hah in the game.
I've seen this mentioned but I've always found his DThrow to be very easy to escape at kill percents. It gets really screwy with rage, too. Do you have any high-level footage ROB's DThrow to UAir being repeatedly successful? I mean, it's a threat, but it hardly seems super reliable.