Jigglypuff if the only character not named Ganondorf that I went back to try to learn for a second time and went "Man, I really overestimated this character last time I used him/her". And I had thought Jigglypuff was solidly in the bottom half of low tier the first time.
She is in fact fairly easy to keep juggling. I don't care if she has great horizontal speed: she's going to get hit a number of times due to her bad fall speed, unless she gets knocked away so far due to her low weight that she's out of range.
Her only realistic/reliable KOing option is Fair or maybe Pound. Drillrest is reliant on a trip, Dash Attack means going to her terrible ground game, Rollout is lol, her smashes mean going once again to her lol ground game, and also have meh range, and Pound is probably used more for spacing. Problem is, Fair is also her best spacing tool if she's facing her opponent in the air. You can try to use Bair/Nair/Pound as much as possible, but when it comes down to it, Fair most likely won't be perfectly fresh when you need it to be. You'll either need to land a few more hits, or hope it hasn't dropped off drastically. And even fresh, it doesn't KO soon enough for the amount of trouble you go through trying to land it. Anyone who knows about Jigglypuff knows that about 140% you'd better start watching for a Fair.
I also believe her ground game's terribleness isn't being stressed enough. I'm honestly not sure who's ground game is better: her's or Ganondorf's. Jigglypuff has crap range, and meh KOing power on the ground, and basically only her grab game with lol bad running speed and Dash Attack. Ganondorf at least has some range to his rather slow moves, flamechoke stuff if he manages to land one, and nice KOing power on the ground. It's close either way, but Ganondorf might edge Jigglypuff out ground game wise due to having at least an average spotdodge.
She's basically usable as a pocket "lol, weren't expecting THIS now, were you?" character against D3, and nothing else.