This is an intriguing question as there are so many players that I loved on the team. Thurman and Bruce were by far my favorites as a young teen in the 90s. Mostly because I considered them to be the absolute best at their positions, but those teams were loaded with stars you could make an argument for any number of players on the team back then. During the drought, Fred and Kyle were of course two guys every Bills fan could be proud of even if they weren't recognized like members of the nineties teams.
The answer for me though is Jim Kelly. He wasn't necessarily my "favorite" at the time, but man when his cancer diagnosis was first reported I couldn't believe what a profound sadness I felt. I was a guys in my 30s crying at my computer reading all the news about Jim and his family. It was then that I realized the impact this person whom I never met had had on me. All the times I shared with loved ones bonding over this team. All the heartache. All the joy. The passion.
Two years later my mother had her own battle with cancer, a battle she ultimately lost. That ***** came fast and hard and she was gone. Her favorite hands down was always #12. Jim's fight became all the more important to me then. Just as on the field, he fought and overcame a lot and that somehow made my loss a little easier to bear. Sorry if this is too serious for the thread.