In it's defence, I would like to mention that I find "Most Immature Member" to be a perfectly valid catagory.

I've been thinking about it, and just incase you don't have enough options (which I can see that you do):

Most Colorful Member
Worst English
Most Likely to Really be a Prince/Princess
Most Likely to have a Sitcom Based on their Life
Most Likely to Swallow a Lightbulb
Most Likely to Have Escaped from the Zoo

I should also mention that I didn't go through these lists and lists of suggestions, so most of mine are probably repeats. Just trying to help.