This was asked in another thread before. Here's the link where you can check out a bunch of muppet fan opinions on their favorite Bert and Ernie skit:
http://forum.muppetcentral.com/showthread.php?t=13084
I posted there that I have so many B&E's I like that it's hard to select a favorite. But I chose one of my all time favorites where Ernie counts to ten and keeps starting over. Among the many other awesome ones, I loved the skits where Ernie questions his existence (after he scares Bert while Bert is reading by saying, "Hi Bert it's time for our checker game" and Bert says he was scared because he didn't know Ernie was there.) and when Ernie and Bert fight over the same chair. The one you mentioned about the cupboard was clever. I remember it only lasting about 30 seconds though. I tend to savor the longer skits a bit more, but there are lots of real great shorties as well. One that just came to mind was when Bert whispers, "This is very quiet." And Ernie says, "What was that, Bert?" Bert says, "I said, 'This is very quiet'." Ernie tells him he still couldn't hear what he said so Bert gets angry and shouts, "I said. Ernie, 'THIS IS VERY QUIIIIETTTT'!!!!"
To which Ernie answers, "That's not quiet,Bert, that's loud. KeeHeeHeeHeeHee"