Which time controls help you improve the most and which ones don’t.

Out of curiosity, which time controls, such as Classical, Rapid, Blitz, Bullet, etc. help you improve the most. I have heard from blog posts how playing Bullet and Ultra Bullet hurts your chess skills, and I just wanted to see what the general chess community here on Lichess thinks.

40/2 then 20/1

Rapid yes, blitz no

Maybe others won't agree with me but i think all tc can help you to improve (even ultra)
Classical helps to calculate, you can give the best of yourself and it's for sure the best time control to use
Rapid helps to calculate a little bit faster
Blitz trains your reflex of recognition of patterns and i used it also a lot to see a lot of openings and to memorize them the more as I can with a lot of games at a reasonable level
Bullet and ultra aren't important but it helps to enter in your opponent's psychology. It's very useful in slower time controls to try to see and understand the psychology of your opponent and i think bullet and ultra trains us with that.

EDIT : But for sure it's a mix. You won't improve your chess while playing only bullet.

I would like to the previous post that an emphasis still has to be put on longer time controls. When I actively played chess, I had to play several LTC games to regain my form, blitz did not help, and rapid did not help enough (back in the days, rapid was 20-40 minutes per game). As the lower bound, I believe that one hour per game (here, it could be 30 mn + increments) is necessary. But then, what do I know.

"Rapid" doesn't give you time to think about your moves as such unless you have increment, i.e. 10+0, if you spend 2 minutes thinking about a move you risk getting flagged later in a dead-drawn position.

Classical is the best, giving you time to think, I think it’s the most enjoyable and the true game, the way it was designed to be played for us mere mortals. Happy chessing!

Improve at what?

You will improve at the time control that you play. Its like the gym. The bits that get used, get stronger.

Having said that, I absolutely agree with the last post.

“... Most internet players think that 30 5 is slow, but that is unlikely slow enough to play 'real' chess. You need a game slow enough so that for most of the game you have time to consider all your candidate moves as well as your opponent’s possible replies that at least include his checks, captures, and serious threats, to make sure you can meet all of them. For the average OTB player G/90 is about the fastest, which might be roughly 60 10 online, where there is some delay. But there is no absolute; some people think faster than others and others can play real chess faster because of experience. Many internet players are reluctant to play slower than 30 5 so you might have to settle for that as a 'slow' game." - NM Dan Heisman (2002)
web.archive.org/web/20140627010008/http://www.chesscafe.com/text/heisman12.pdf

Bullet doesn't "hurt" your chess skills.

The only thing that "hurts" your chess skills is not playing at all.

The only reason people recommend classical for learning, is because it forces you to learn the theory.

Just play stockfish for 20-30 minutes, then analyze the game. Learn from mistakes.