"πόλη" vs. "κωμόπολη" - one is smaller than the other, or are they synonyms?
I always think of κωμόπολη as being either a small town, or a large village.
I don't think so. There's a preposition in there, σε/στην, which makes the sentence something like at the/on the/to the/in the/by the, none of which work with the idiomatic usage. The idiomatic usage - which I've only come across here on Duo, in one sentence - doesn't seem to require an article at all, so the sentence would be "Η κωμόπολη είναι κορυφή".
the suggested option 'the best' also fits but was not accepted when i gave it as an answer