No Prep Time
Each at full power
I'm going to give this to Minato. He's simply too fast, he'll analyse how Pein's jutsu works realising it takes him about 5 seconds before he can use his shinra tensei as Minato was one of the most intelligent ninja seen in the series.
Seeing how Sage Naruto went to town on Pein and was completely decimating him before he ran out of Sage Chakra, I'd say Minato, who is much faster than Sage Naruto, would easily be speed blitzing Pein.
Of course we haven't really see what Pein is really capable of when his power is all within one body so I'm using the 6 paths of Pein here.
IMO this fight might become a stalemate only because Pein's Shinra Tensei leaves a 5 second opening for anyone to attack, and we all know Min is no slacker... Hiraishin can counter all the paths but the only problem is that they all share the same field vision meaning Minato is going to have to think more strategically before flinging a kunai at any of them.
A five second gap for Minato would be like an eternity. This is the same guy, after all, who made the Raikage look slow. The shared field of vision just wouldn't be able to react, look at all the good it did against Sage Naruto who nearly killed Pein and would have killed him too if he had not run out of chakra. Minato is leagues faster than Sage Naruto and can effectively teleport with his Flying God jutsu. The only problem I forsee here is Pein using his Shinra Tensei to blast away all of Minato's kunais but then Minato could just throw down some more. It's just a pure speed blitz and unlike Itachi, Pein doesn't have the constant effects of Susanoo to bail him out.