List of designers who know what they are doing(and those who dont)
Im keeping this list to more recent champions(thresh will be the oldest champion ill use, and thats still over two years old). Also im focusing primarily on design not wether something is under/overpowered. For example, i think zach is an extremely well designed champion, there were times where zac was pretty overpowered, and times where hes been pretty bad. But at his core his design values are not bad because he has discernible weaknesses and strengths.
Disclaimer- I believe league of legends is supposed to be primarily a strategy game, and not a button mashy/reaction time based game. To some degree it has to have both in order to be enjoyable, and the problem with how riot is designing their champions(which isnt entirely their fault because its not like they could see the future and see that they were going to change what they wanted the game to be like) is that skill cap differential between champions is way too severe, especially in regards to older vs newer champions.
Its sort of where the idea of "outplaying" someone comes from, when in reality if yasuo is good enough theres nothing garen can really do regardless of how skilled they are(in contrast if both players are bad its easier for garen to do better). In my opinion there needs to be some happy medium where strategy is allowed to overcome skill, and the problem is about half of the new champions fall in line with that, and the other half dont.
The two worst designers in regards to this are CertainlyT and gypsylord. Pretty much all of their champions are designed with the philosophy of being able to outplay the other person and having extremely high skill caps/steep learning curves. Which would be fine if every other champion in the game was like that, except THE VAST MAJORITY ARENT. Almost every champion theyve made(thresh, kallista, ekko, gnar, yasuo, jinx) has, if you are good enough absolutely no weakness. All of those champions with exception of thresh have mobility, all of them have hard cc, all of them have high damage(for a support thresh's damage is pretty decent), most of them have random defensive stats on their abilities that allow them to be tankier than that rightfully should be. Also a shoutout to zennon. Azir has all of the problems i mentioned above(soft cc, hard cc, random defensive stats, mobility, high damage), and lucian for the longest time had no discernable weakness within his role until they gutted his range, and still remains overall a pretty jack of all trades adc, he just pales in comparison to jinx in seiges/teamfights which makes him seem weaker than he really is.
Probably the best designer as of late would be subninja in my opinion, and a lot of people that have only designed one champion(such as bard's designer rabid llama, braum's designer riotwreckz, aatrox's designer classick or reksai's deisgner beat punchbeef). The reason why these champions i consider are healthily designed is they have a purpose, and they are all discernably vulnerable in areas of their kits.
Bard- doesnt really have that much hard CC for a support or damage, but brings a ton of unique utility to the game which can make him a good pick in certain situations.
velkoz- the epitome of a long range mage champion who does a lot of aoe damage, similar to brand just different ability mechanics.
braum- Doesnt have a lot of damage, his mobility is extremely limited. Is much better as a peeler than an initiator.
reksai- The only one of these thats really debateable, but her cc is really minimal, and while her tunnel mobility/early game damage should be toned down a little in my opinion she doesnt have a bunch of unecessary shields or stuns or slows. Very much a map pressure oriented champion.
TL;DR Riot i know you have good designers, please use them. Or at the very leash assign some of the ones i listed to review certainlyT/gypsylord/zenon's work before they release them to make sure they dont make inheritly unfun and difficult/impossible to balance champions.