I'm still wearing briefs but all the other boys in my gym class wear boxers. Which one would be the best one for me to wear? Is that a problem?
Fashion, even in underwear, changes over time. I remember in my own childhood that briefs were common and only a few boys wore boxers. It appears the tide has turned again.
Since underwear is not for public display, you should pick whatever you find most comfortable to wear. If you live in a warm region of the world, you want something that doesn't hold moisture near your genitals as that can lead to jock-itch problems. Otherwise, try a few different styles and see which you like the best.