So... ADHD is a late night programming block that started on FOX in 2013, and moved to FXX in 2015. But I've pretty much only ever watched it online. The block consists of several shows, as well as interstitial shorts. I think at least some of the shorts originated online before airing on TV, but I have no idea if that's true of all the shorts, or just some of them, or maybe even none of them. So... I'll include links to this page in both my webseries section and my TV microseries section.
Anyway, there are over 100 random shorts, I guess, and I haven't watched many of them. Honestly, I don't have much interest in them, but maybe some of the ones I saw weren't bad. Some were definitely lame, but I thought Goodbye, Ghibli and Sympathy for Slender Man Song were pretty decent. Also, I watched something called Schoolgirl Crush, which was okay. But then like a lot of time later I checked out the YouTube channel again and saw that it was now a series (5 episodes), so I watched the ones I hadn't seen, and it was kind of fun I guess.
Maybe I've forgotten some other shorts that were decent, but what I would recommend for you is just to look through the lists on one of the sites linked above (it's pretty much the same vids on each site, I think) and just try any short with a title that sounds interesting to you. (If you try a short and don't like it, you've only lost a couple minutes of your life, no biggie.) However, in addition to all the random shorts, there are some specific webseries (or microseries?) I'd definitely recommend. Not necessarily because they're so awesome (though they're reasonably amusing), but because they relate to specific shows on ADHD:
The Adventures of OG Sherlock Kush
IMDb; Wikipedia; YouTube
OG Sherlock Kush is a brand of weed the Lucas brothers smoke in Lucas Bros. Moving Co. But in this webseries, OG Sherlock Kush is a Sherlock Holmes-like detective, except more drug-addled. Which is funny, because we all know Sherlock Holmes is a drug addict, so seeing an animated detective who has a bong with him at all times is weirdly appropriate, even if it's taken to absurd extremes. (Also, damn do I feel bad for Watson.) And I'm not sure, but my guess would be that this show, and its protagonist, is probably like an advertisement for the weed, in much the same way that Popeye cartoons were basically an ad for spinach. Maybe. Whatever, it's redonk, and it's not as good as the Lucas Bros show, but it's alright. Also it's short, which is a plus. (Though there may be some episodes I haven't seen, that were added after I thought the series was over. I should check, someday.)
A sort of surreal webseries about the cat from Stone Quackers. Except, on that show his name is "Gothfield" and in this webseries his name is "Gothball." So that's weird. But anyway, even if I like his name better on the TV show, in general I kind of like the webseries better than the show itself. The animation is cruder, but it's told from the perspective of Gothball, like we hear his cat thoughts, which we can't do on the show. And it's funnier than the actual show, in my opinion. Also it's shorter, which is a plus.