I think the rarest plat I have is from a racing game (Sonic and Sega All-Stars Racing Transformed), but I don't feel like it was the hardest in comparison to some of the other stuff I've gotten.
I had a harder time on Trine 2 I think.
But the hardest plat that I tried to get but FAILED at getting?
Super Monkey Ball Banana Blitz HD. For me, that's probably the pinnacle of gaming difficulty. I don't think I'll ever be able to 100% that game.