Oh gosh, so many to choose! I loved Impmon/Beelzemon for being such a redeemable character, but I also really liked Ruki and Renamon. I love how their relationship developed.
This is a very difficult one, because i think Digimon Tamers has some of the best characters of all the franchise.
But my favourite are Takato because i love his evolution, and terriermon because of his philosophy of life.