Kaguya's nine tomoe Rinnegan sounds like the most powerful, since it can cast Infinite Tsukuyomi and Dimensional Shifting. About Sasuke's six tomoe Rinnegan and Hagoromo/Madara's purple Rinnegan, I think they're tied.
This is my take on the whole Rinnegan mess. There's just 2 versions of the Rinnegan:
The color I theorize simply indicates if the eye is focused on Ninjutsu or Genjutsu. Only red Rinnegan in my opinion can cast Infinite Tsukuyomi, while only grey Rinnegan can use Six Paths Technique.
For Sasuke's 6 tomoe, in my opinion he actually has 9 tomoe, since the other 3 are in his Sharingan, so his Rinnegan isn't missing anything by having 6.
Therefore Sasuke's Rinnegan is on par with Kaguya's in terms of "evolution", just is Ninjutsu focused like Hagoromo's.
EDIT: if Sasuke's other eye were to evolve into the Rinnegan, it would be red, that's my theory anyway.
i think the strongest is kaguyas, simply because 9 tomoe and rinnegan -_-
next is sasukes because of 6 tomoe and rinnegan -_-
than madara/so6p with 0 tomoe and rinnegan o_O
than EMS, MS, 3 tomoe sharingan, 2 tomoe sharingan, 1 tomoe sharingan, completly normal eyes...
i wonder if we will also see a "3 tomoe and rinnegan" :/ it would be between madara/so6p and sasukes (like the sharingan with 1,2,3, the rinnegan has 3,6,9 :D )
1. Kaguya's 2. Sasuke's 3. Madara's
^ thought so
Also this thread is almost a year old, I don't encourage further discussion.