I understand that nobody wants to share their winning models because then they won't be winning any more. So I think there will be people who will have no problem to post their losing models, just to get some discussion going. A system qualifies as "losing" if it has been bet unsuccessfully or if its out-of-sample prediction is unsuccessful.
I will make a start:
Elo and Glicko systems on tennis. Tried to optimize the parameters using brute-force optimization and also Nelder-Mead optimization. Out-of-sample prediction very lacking.