I would say the International Sesame Street Muppets from the 1972 version of Vila Sesamo are the ugliest.
First, there's Garibalo, the residential bird of Vila Sesamo. He seems to be a rip-off of the original Big Bird from the American Sesame Street. Gugu, the grouch, seems to have a creepy and ugly face, which would scare little children.
Heck, is Funga Funga, the red elephant, supposed to be the Brazilian counterpart of Mr. Snuffleupagus?
And last but not least, there's Ze Das Latas, which means "Ze of Cans" in English. He's the robot of Vila Sesamo. Doesn't he look like the CDA from Monsters Inc?
Overall, Vila Sesamo seems to be the ugliest co-production of Sesame Street ever. And please, don't look at any Vila Sesamo pictures or videos before going to sleep. They will give you the chills.