While having a competition of some sort would be fun, I think that due to the diversity of the machines built and shown on this forum, it would be nigh impossible to decide a "best" kart every month.
If a contest were implemented, there would have to be several categories for the many different uses we have for the stuff we build. Offroad, Buggies, Shifters, scratch builds, etc.
I do like Robin's idea though. If we could use the type of ratings system on cardomain, for example, that would be cool. Have several different points of interest that could be rated.
Frame
Suspension(if applicable)
Tires
Engine(mods?)
Drive setup
Speed
Paint
It would be kind of cool to see what people think about your creations. With a rating system, more people are likely to click to rate, as it's easier than typing out a review. I know I enjoy reviewing and rating other people's builds, and I'd like to see what people think of mine.
Just some food for thought.