It's tough to decide. Gilchrist was the best all around player, absolutely no question, plus he won a championship. However, he was only with the Bills 3 seasons, so I'm not so sure. OJ was the best pure runner, Kelly the best QB and Smith was the best overall defensive player but none of them produced a title. Thomas might also merit consideration, but again, no championship.
I'm going to go ahead and say Jim Kelly. He never played for another NFL team and was the face of the franchise from the day he arrived, for 11 season. He led the team to it's most prolonged period of success and although never won the big one, he did help the team get there 4 times and played pretty well in two of the four games (and left a third due to injury with his team in the lead).