2. The One With the Holiday Armadillo
In this classic season 7 episode, Ross has his son Ben over the holidays and wants to teach him all about Hanukkah. A visit to the fancy dress costume however reveals that all Santa costumes have sold out, along with anything else remotely Christmassy. So Ross becomes The Holiday Armadillo!
We also see Chandler dress up as Santa (and Monica reveals a rather disturbing fetish), and Joey as Superman.