Is Master Yi most powerfull Champion in LoL?

no lawl, he's easy to play but no to the point of being the best champ in the game

master is a champion that can be easily stopped so you can not play on silver

It depends on the division, in bronze he's really easy to hard carry but in challenger you never see him

Not the most powerful, it's just the matter of build and the timing. Even if he's over fed, heavy CC and bad engage can turn the tables of the match.

Plus, in higher leagues, they know how to counter him easily.

Balanced. Fun. Has a dedicated rol with a decent kit without being overloaded-- I like him.

definitely not, he is one of the most snowball-y champions though. that probably makes him feel very strong to someone who doesn't know how to shut him down.

If your criteria of which champ is more powerful is: 'hitting a stationary target and measuring DPS output' then YES he's arguably the best champion.
What makes a champ powerful is however much more than how much dmg he can dish out. How reliably can he dish that damage out, how tanky is he, what CC can a champ bring, what else can he bring to a TF.

With that in mind, I'd say no, he's not the most powerful champ (not even close because he only offers insane dmg) but is definitely powerful (with major downsides - squishiness, need to farm, little help to his laners since he's farm heavy, cucked by zhonyas etc...)

YI is very strong but at the same time he is very fragile

At the moment Yi is the strongest jungler in the game and is quite useful