Personally, I like buttons that are witty/tell a good joke. I'll admit, I may not remember the team number on it, but I'll definitely remember the person who gave it to me. Have enthusiastic (but professional) and well-decorated hander-outers, preferably that say something ("hi"* works) before asking if you want a button. It's all in the presentation!
*"Hi, I'm from Team XXXX, that's a cool robot you got there (I like YYYY). Would you like a button?" works even better.