Definitely the unicycle convention. I think that the other ones are cool though don't set themselves apart enough from other novelty tee-shirts, especially with the text on the lovers one. Nonetheless, I would still wear them!
My favorite b minus shirt is definitely "bike <3". I like this because my regular commuter is an Xtracycle-equipped Surly Big Dummy with a custom double-child seat snapdeck replacement. Since I ride with my two kids all the time, the t shirt would be very appropriate for me. Our Big Dummy is mathematically less then the three of us.