Bills and Panthers are no surprise, so for me it is between the Lions and 49ers.
I thought the Lions would do a bit better this year, but they have played some tough opponents in the first four games and have played relatively close games. Of course the Stafford injury hurts.
My guess is many analysts are shocked by how the 49ers have started. I recall many analysts describing the 49ers as a playoff team and perhaps even some mentioned them as superbowl contender. But I never thought that, so I'd be lying if I said I was surprised by how bad they've been playing. Why? I think a lot of it comes down to how poor of a QB is Alex Smith. I really think that franchise would benefit by just cutting the guy and starting over at the QB position (like Oakland did).
So my vote is for Detroit.