Is yi the most broken champ ???

I don't think so. Yi is really vulnerable in the early game. If he gets three-to-four items, that's when you should be a little bit scared. If you punish him/invade him in the early game and you succeed, then Yi will have a really hard time to get back into the game and get the resources he needs. If early game doesn't work out for you, the two options in mid-to-late game (when he has his items) is heavy CC (crowd control) in fights. For example; Shen's E (taunt), Nautilus' Q (stun), W autoattack (root), R (knock up), Blitz's Q (hook) + E (knockup + R (Silence) etc.Whenever someone from your team catches Yi with some form of CC you should switch your focus onto him and quickly kill him before he gets to do anything like pressing W which mitigates damage or pressing Q which will enable him to delete your team. The other option is catching him offguard. For example when he is walking to Baron river and you flash + CC him and quickly kill him.