I know this has been suggested but not sure Tapas is considering it.
If they were to run a contest for existing stories it would be a different type of contest from the writers camp. And I think it would be unfair, unless premium stories are excluded.
Perhaps a yearly best off contest -
Best Romance, Best Horror etc. and readers can vote for their best and the top five from each category are then voted on for a winner. (To make it fair Best Romance - Premium and Best Romance - Non premium).