What is the best time control for improvement?

I heard somewhere that Classical helps you the most but you can play like 100 bullet games with that time

Rapid is the perfect balance in my opinion.

But I can play like 10 bullet games with that time

I was told an hour, maybe it depends on the person? Chess is the game that makes your heart sing with joy and scream in pain in the same game at times!

Sure. You could also play 60 10 second games in that time as well. To each their own, bullet is more of a fun pastime then a serious way to improve though. And it’s a bit of a trap, since you can play so many games you might waste a lot of time. Bullet is probably only useful at the top level since it prevents cheating but also gives much less drawn games between grandmasters, blitz does the same though.

RAPID

If you train for long distance then you will get better at that. If you train for sprint you will improve at sprinting. What do you want to improve at?