Scoob ... I think you have your work cut out for you. Imo the trouble with "frostiest bud" or equivalent is that it's subjective, and with a prize in the balance that could be a real wedge driver. Same problems with essays or something creative ... one man's Joyce is another man's boxcar graffiti.
A merit-based contest might be easier to administer, because it would be easier (but not necessarily obvious) to designate a winner and defend the choice. But ... merit at what? Weed growing? I'd counsel against that, with my sufficient rationale being weed's less-than-transparent legal status. Gardening otherwise? My worry is that that would bias strongly toward the small number of skilled, experienced gardeners like we get to meet on the gardening subforum.
Now my weakness for recursive thought has suggested to me "what if the undisclosed contest is 'come up with the best contest idea?' " That is attractive to me at so many levels, but I'll have to add that I've probably already lost.
cn