Fisher-Price's Oscar the Grouch is so simple yet so effective and I still to this day love popping him head up and back down into the can.
This isn't my original. I have long since lost all of those. This one came from the flea market, and I probably paid a quarter. Oscar would be proud!
I grab these guys from the flea market etc whenever I spot them (for cheap), which isn't as often as I'd like. I only have a few right now, but Oscar will always be at the top (of the trash heap!) as the coolest of them all!