Juggernauts are the best designed champions in League

You can't generalise like that, you have to look at the champs individually. There are badly designed Juggernauts as well as well designed ones.

Example for a badly designed Juggernaut:

The general idea of a champion that is highly tanky and deals alot of damage, but being kitable and therefore the damage is unreliable might sound like a fair trade off. But not necessarily for every elo (so level of skill).

Kiting is a skill you have to learn and if a champions weakness is not lacking a stat (like marksmen lack defence or tanks lack offense), then such a champ can easily become a "pubstomper". Which I wouldn't necessarily define as the "best designed class".

In general, the ideas of the subclasses by Riot all sound fair. Marksmen have high and reliable damage, but without help from their team they instantly blow up against pretty much every other subclass.

Divers have a bit of everything, but not as much as the specialists. They are tanky, but not as much as a vanguard or warden. They got damage, but not as much as a marksman. They have mobility, but get beaten there by assassins.

The general concepts of each subclass sound fair for me. What's more important is to look at each champ individually if it is well designed, instead of declaring a single class as the best designed one.

It's impossible to have a proper discussion on this based purely on the OP, because the OP doesn't really explain what standards for good design they're setting in this case, and what qualifies as being "too well designed". It's implied that the standard is having clear strengths and weaknesses, but that is generally only one out of the several pillars of good design in League.

With respect to clear strengths and weaknesses, I also feel like juggernauts are a prime example of how one cannot rely exclusively on that design pillar to produce good champions, and how going too ham on that can in fact be bad design in the long run. The core issue with juggernauts is that, because their strengths and weaknesses are so extreme, their playstyle frequently ends up being just as binary: on equal power, if you fight them at melee range, you lose, but if you get to fight them out of their range, which can be done reliably by most ranged champions and more organized teams, you win, and so with little room for nuance or variability in-between. If juggernauts had more counterplay at melee range, but also more tools to affect enemies at a distance, it would render their strengths and weaknesses a tad less sharp, but would come at a tremendous gain in agency and interactivity, which I think would make juggernauts a much better-designed subclass overall.

I would disagree. Illaoi is insanely frustrating to play against. Garen is very binary to play against. Darius is . . . Darius.

Well Jugs are a good design the problem is the game itself and the dev's dicking around. Thing is about juggs is that they have to play much smarter than other classes as they have poor skill expression and outplay potential. Hell that is the main reason I play them, because they are simple but powerful which makes them strong front liners.