I think Pepe is probably the most infectious Muppet voice to do. Once you get the basics down, you will find yourself using that voice to synthesize new lines, not just the ones you've heard him do before.
Once you start, is really hard to stop, hokay?
But Link Hogthrob and Beauregard are probably the two that I have 'quoted' the most often. Link's "You were in the driver's seat; you forgot to push the stoppy-thing!" and Beau's "I'm sorry. I can only take you as far as the lobby!" just seem to pop up an awful lot in my everyday conversation, whether they're apropos or not.
And this one is obscure, but I have really been running it into the ground... in "The Muppets on Puppets", during the muddled fairytale, Taminella Grinderfall (Jerry Juhl) asks confused narrator Rowlf, "What's the matter with YOU, doggie?"
If I see a worried look cross the face of my wife or my kids, I always slip into Tammy for a moment and express my concern disguised in her abrasive way. In fact, I have to stop myself from using the line at work, where I know no one will catch the decades-old reference.
DN