I could never choose. I love the main five best, and there's no way I could possibly narrow it down further than that. Fraggle Rock has been my favorite TV show ever since I first saw it as a little kid in the eighties. The main five are like family to me. ^_^
When I first started watching the show, however, I had two favorites: Red and the Storyteller. I was pretty much obsessed with them.
I now have a certain fondness for Mokey and Boober, as they were my mom's two favorites. She loved Mokey's singing, and Boober always made her laugh. She passed away in '09. I fondly remember her reactions to Mokey and Boober with each episode I watch.
FYI: Beige, a one-time character, is not a favorite. I've just always liked his name, so much so that if I ever had kids, I would name my firstborn son Beige. It's just a beautiful-sounding name to me. ^_^