This is me guessing/being cynical
Pretty miserable:
Falco (to some extent Fox)
Link (smaller extent Toon Link)
Sheik
Doable, but tough:
Snake
Ivysaur
Rob
Wario
Basically, in my opinion, Ganon hates two things - ways to dictate the neutral (often with projectiles) and ways to ensure he's dead once he's off stage. Sheik and Falco have both of these, with Lasers and Needles, as well as a spike for Falco, and Sheik's amazing gimping ability. Fox rips you open because of a better neutral, but you also don't need to deal with lasers, bringing the difficulty down a bit, just don't get caught in shield - Ganon's OoS game sucks. Toon Link I find is less projectile heavy and has a slightly worse recovery than Link.
Characters in the second section often only excel at one, or they are good at both, but not great. Snake can zone with his entire kit and is decent at edgeguarding, but is very space reliant - a smaller stage makes this matchup more exciting. You're dead to bair offstage against Ivy, but she's light and easy to punish due to her B tether. Rob's got it all - weight, projectiles, recovery/gimpability, but if you manage to keep the space between you two to a minimum, and bait/punish boost canceled aerials, you negate half of ROB's kit. Wario is just unpredictable, and is amazing at edgeguarding Ganon, but his recovery is linear enough that you should have him dead if he's off stage.
Ganon doesn't have many great matchups, as stated by Cory. Instead, you wanna focus on playstyle vs. playstyle, player vs. player, and make the right call. I would only resort to a secondary for matchups you personally struggle with - I like having Peach for fast fallers, for example, and once in a WF's side of a tournament I counterpicked a Charizard with my DK to make it to grands - not because it was a bad matchup for Ganon, but because I (correctly) guessed that DK's combo game on Charizard would be more lethal than Ganon's "combo" game.