I really dislike playing against Pikachu personally. If he plays right, he's nearly impossible to edgeguard, and can't really be juggled outside of low percent followups. Not to mention because he's so short, it's really hard to hit him with any aerials in general.
I think my personal hardest matchup currently is Duck Hunt. The only missed tech choke combo you have on him is Jab, and he can cover his landing safely with the Clay Pigeon if you are not directly underneath him. Plus I honestly don't know what the best way to deal with the Can is. The times I have won this matchup are ones where I manage to either establish a techchase rhythm on him or edgeguard him to death.
After that matchup, Luigi, Pikachu, and Rosalina players I feel have been generally been the most difficult for me to deal with. They camp, it's a headache to avoid getting grabbed in those matchups, and they can all punish Ganondorf very hard when it matters.
I also find Sheik extremely difficult by design, but I feel like practically nobody that plays her on For Glory really knows what they're doing with her and I clutch out wins with Ganon's superior KO options. Not too many people play Robin, but I find him really difficult if he land traps correctly. Mega Man, Greninja, Villager, Sonic, and Lucario are very tedious, but doable as long as you know the right situations to avoid putting yourself in.
Everyone else I didn't mention here I currently feel Ganondorf likely has an approximately neutral matchup with on default settings. In my limited observation, most things seem winnable for Ganon as long as you avoid getting gimped too early.