Is Master Yi most powerfull Champion in LoL?

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...)