I'd say the Rinnegan is the strongest, but all three are pretty powerful. The Sharingan wasn't created by the Byakugan, as far as we know.. I think it was Kakashi who said the Sharingan was rumored to be a mutation of the Byakugan. Really though, it comes down to what the Uchiha Clan Ancestor's eyes were. If he had a Rinnegan, then they both likely came from the Rinnegan. If he had some sort of Sharingan/Byakugan fusion... then that might be it. Then again, the Hyuuga being a different clan might have evolved it separately on their own. Who knows?
The Rinnegan is beast though. We know that it can utilize all six types of chakra natures, we know that (at least Pain/Nagato could) see barrier-type techniques (Which are probably made of chakra, so it would be safe to assume it can see chakra as well), and we know that on two separate accounts, a possessor of the Rinnegan created techniques that could copy bloodlines. Nagato, as the Rinnegan user, has shown the ability to instantly kill people by removing their souls, revive the dead, and copious use of gravity or attraction/repulsion based techniques.
The Sharingan can copy any technique that isn't bloodline, has a natural powerful genjutsu, can see chakra flow, has a form of precognition and extreme eye for detail and clarity. Plus it's got the MS techniques and EMS.. whatever that does. Finally, it has Izanagi.
The Byakugan can be pretty beastly too though. 360 degree vision, able to see upwards of several hundred meters away.. x-ray vision.. practical perfect chakra sight.. all for one measly little blindspot. It's a shame that the Hyuuga thus far are almost exclusively taijutsu specialists.. a ninjutsu or a genjutsu specialist with the Byakugan would be something fierce to face in battle.
I still think the Rinnegan is strongest, however.