Oh boy, this is going to be long...
@Guavo
@chaos11011
YerTheBestAROUND
Jamurai already explained the Falco match up.
Charizard has significant strengths against Doc & vice-verse.
Doc's neutral is very underrated since he can auto cancel his short-hop B-airs for spacing & he has pills which forces Charizard to approach. Doc's amazing frame data gives him the edge in defense, especially out-of-shield. Zard is easy combo food, through Doc shouldn't get greedy since Zard can amour with Rock Smash, Doc's kill power is similar to Zard's so it's not hard for either of them to get a kill & while it's difficult to edge-guard Zard, a well timed Cape or B-air can be enough to end Charizard's stock.
So IMO, the match comes down to who can outplay who.
DHD's zoning can be invalided by Doc's Cape. (No pun intended)
Unless DH has a can with them, it's very easy to edge guard them with Doc's B-air, D-air, Up-B or Tornado. Doc also doesn't have much trouble getting a KO on DH due to his fast frame data & kill-power, while DH struggles to get a KO without the use of a read.
This match up is by not means impossible for DH manly because they can escape Doc's combos, (Huh, there it is again) but they definitely have to work into order to get that edge over Doc in the match.
Anything Luigi can do Doc can do back, simple as that.
DK loses in neutral since he's forced to approach in which case Doc usually wins in defense, he's easy to combo off of, & it's easy to edge-guard him. However DK has Ding-Dong which is amazing for racking up damage & killing.
While Fox has a slightly better neutral & greater kill power, he's can be combo easily, light & Tornado eat through Fox's recovery options.
Falcon can combo, kill & edge-guard but so can Doc.
Aside from his disjoints, Pit & Pitto don't really have anything crazy they can use over Doc. But for reasons I've stated above they are not safe from Doc's edge-guarding ability.
I actually don't know much about the Greninja match-up, so that could change for Greninja favor.
Diddy's Neutral can be rival by Doc's. (Throw in everything I said about Doc's general strengths) However, Diddy's fast frame data & quick kill confirms are enough to win the match for him.
Marth, Lucina, Sonic, Rosalina, and Corrin are not impossible to win but they just bad enough to were playing Doc is awful idea. (Even then, can challenge some of their weakness)
*GASP* Man that was a mouthful.
Well I hoped I explained my reasons in a clear manner.
Can I get a gold star?