The correct term is Guapérrima, surplus one R. It is a way of showing that someone (woman) is excessively beautiful. I would say best beautiful or beautiful.
It's a morphological donkey, which only goes as an exaggeration by way of a joke. It would be "beautiful" (feminine of handsome) with the suffix _ érrimo in preparation to make a superlative and a "r" tripled to reinforce the idea.