I have 3 DC - the first has ADHD. I was also a complete mess with my own undiagnosed/unmanaged ADHD when he was little and it was a huge chaotic storm of crapness. I couldn't get him to do ANYTHING. This regularly reduced me to tears because I felt like "WTF, he's 4??"
Anyway he mostly grew out of being a little butt
I think he would have been OK but a bit of a PITA if I had actually been consistent about things plus leant into some autonomy focused stuff (which I did TBF and which did help a lot, but my own chaos was the thing which dragged it all down). He really knew how to push your buttons, well especially mine. He is lovely now even if his thought processes are a bit sideways sometimes (I put this down to teenage hormones TBF) and I truly believe he was never actually trying to ruin my life much as it felt like it when he was between the ages of 3-7 particularly. However, reward/punishment still absolutely no use and never has been. He just feels hard done by and doesn't connect his action to the outcome, because his actions always make sense to him even if he's clearly BSing himself an excuse right there as the words come out of his mouth. If it's directly related, it makes more sense to him, but that's it. This is for school as well as us.
DS2 has ADHD too but we think he is also autistic. He started medication much earlier, because we could get him to do things at home since we knew what worked for him - a little autonomy, but a huge amount of routine/understanding what would happen when and why, but school was an absolute nightmare for him and he couldn't cope with the unpredictability of the 25 other people in the room, and would completely lose the plot. Medication has transformed everything, although it's still true that it would be extremely difficult to get him to do anything using reward/punishment/disapproval. It just doesn't work unless his agenda is taken into account. What the medication does is reduce his general sensory overwhelm and allow him the brain space and pause button to consider other people's agendas (rather than just imploding at the first sense that someone else might have one) and he may or may not accept them alongside his own, TBF he usually does now that he actually has the ability to consider them. In many ways his adherence to patterns/routine makes him the easiest child, but OTOH it means you can't really make exceptions "just this once" as he will then understand this to mean "this is the new routine" and melt down if that doesn't happen, unless you can summon sufficient enthusiasm/energy to make up some kind of complicated reason behind why we can do it this time but not every time. DH is great at this, and it works for school, because any time they break the routine it generally is for some special event and they make some fanfare about it. But I am utterly terrible at it and tend to forget my own rules plus what the normal rule is and DS2 gets enraged at my inability to just follow the plan 
Anyway the reason I wanted to reply to the thread is that DS3 is in that peak butthead kind of age (4.5) and although I often find him utterly infuriating he also just responds to everything including discipline completely differently to the other two. For example this evening DH called him up for his shower and he replied "No, I don't want to have a shower!!" DH said "You have to have a shower" and DS3 said no again. I braced myself mentally because I just knew I was going to have to go and carry him to the shower or invent some kind of game to get him upstairs but by the time I turned around I realised that he was huffily stomping up the stairs saying "I. Don't. Want. To. Have. A. Shower!" with every step!
He then proceeded to get into the shower and complain throughout but did get himself washed. This is also the kind of thing I've seen DC do in other families - ie, do the thing they are supposed to do, even if they are complaining about it, and I wondered if their parents were doing something that I was missing, but I don't think that's what it was. DS1/DS2 would absolutely never have gone towards the shower with that depth of feeling about not wanting it. You would have had to drag them in kicking and screaming or employ elaborate persuasion tactics to convince them that it was their own idea, and with DS1 it was like that every single time for fifteen years until he suddenly wanted to live in the shower and spend £££££££ on expensive toiletries. DS2 was easier because as long as the shower/bath is a routine he will happily stick to it.