I thought καλητερο was the comparative, not the superlative. Was I wrong?
- καλύτερο = better
- το καλύτερο = the best
The modern Greek superlative is the same as the comparative but with the definite article in front of it.
(This means that it's ambiguous with "the" + comparative -- το καλύτερο could also mean "the better (one)".)
To add: there is a form of the superlative that is different, "-τατο," but as far as I can tell it is less common than just using the comparative. Further, there is the irregular superlative of καλός which is άριστος, which may be a little archaic but I think is still used pretty well.