byukugan is the weakest
It depends on what it is used for everybody!! You cant say 1 dojitsu is weaker or stronger than another. The Sharingan is used to see past genjitsu and have abilities to predict movement. While the Byukugan can see a large radius, aswell as see chakra flow and the chakra holes on a persons body, we are yet to see the Renningans ability but we know that they can see from different bodies, which makes it even more dangerous than the Sharingan. It all depends on how good your eyes technique complements the chakra element type that you use and complements your fighting style.
Like Lee said, what good is it that your eyes can see me screwing up your face, and your body cant do anything to prevent it.
If you have placed Neji in the chunnin's preliminary exams against Sasuke I think that Neji could have beaten Sasuke, not just because of the Byukugan but because of how his fighting style complements his eye technique. Sasuke relied to much on the Sharingan instead of developing a way to train his taijitsu to assist his Sharingan and complement each other. He however did learn more hand to hand skills, later prior to his match with Gaara.