Well, there were so many for me, but the truly special ones were... well...
My favourites were Corey, as it was the first one when I felt like the stakes were really high in the game. And also due to the fact that I managed to solo him (non-mist) with Emolga XD, and Shade, because that's where the plot truly started to develop, and also because Shade is perhaps my favourite character in Reborn.