https://youtu.be/ct9y9lDX990
https://youtu.be/Rtjur1BZq44
https://youtu.be/lUC83_6Wa5A
https://youtu.be/UCSjBXle-Yg
If we ignore the non canon chinese comic. Mr.X always felt like he was on pair with William's peak forms once he transformed into his super tyrant form in the original. His regular form is a pushover but his super tyrant form gave him a massive power boost compared to the power boost William got with each mutantion.
The G virus is superior but I see it this way. Mr. X is a Umbrella certified, lab tested, enhanced, and optimized T virus being. While William is a glitchy, buggy, and unoptimized G virus being.
Giant Moth -200 pts of power
G Spawn - 900 pts of power
Mr. X - 900 pts of power
G1 - 1,000 pts of power
G2 - 2,000 pts of power
G3 - 3,000 pts of power
G4 - 4,000 pts of power
Mr. X Super Tyrant - 4,000 pts of power
Giant Alligator - 4,000 pts of powerG5 - 5,000 pts of power
William peak at either G3 or G4 but the thing is G3 can be forced to retreat and G4 can be knocked out/killed temporarily while Super Tyrant X cannot be stopped by any sorts of small arms fire nor can he be forced to retreat and still attacks in a professional manner in it's super tyrant form.
Giant Alligator is also a good contender for the most powerful because of its size and deadly bite.
Giant Alligator, Super X, and G4 is a good matchup that could go either way.
If you ain't a Fiend, you ain't Clean!