His argument was Yoda, given he commands the powers of the universe, and even in death, transcends the mortal realm and becomes only stronger. Also, lightsabers cut through everything.
After a while, I submitted, as the writing of Yoda's character is too well crafted. Master Chief's bullets are worthless, Reapers from Mass Effect would likely be unable to exert mental control over him as well as their hulls being easily breached by a lightsaber, Superman is weak to kryptonite as well as being unable to punch the super agile Yoda, Sephiroth's sword would just get cut in half, Gods were killed by Kratos and Kratos would arguably get destroyed by Yoda in a one on one.
Hell, we've seen Yoda single handedly wipe out armies with just his brain. In addition, there exists many other potent Jedi spread amongst the entire universe, in which Yoda would be able to call on in an instant.
He commands the universe's force like no other and has the power, skill and technological edge that makes him the end all, be all most powerful fictional character ever.
So I challenge you, present a case as to how there exists another fictional character that could truly defeat Yoda.
TL;DR you can't think of a fictional character more powerful than Yoda.
Edit: Can't splel