Honestly — any time of the year.
Paphos is worth visiting in every month because Cyprus gets plenty of sunshine and there is always something to do.
Summer is perfect if you want hot weather, beaches and long days by the sea but have in mind that is means that you will have to survive even 8 Celsius degrees…
Spring and autumn are great if you prefer warm weather with fewer tourists and don’t like to melt 😉
Winter in my opinion is the best 😉 For sure is the cooler, no heat waves, occasionally it even rains. Of course you can still explore Paphos, visit archaeological sites, walk along the harbour, enjoy local restaurants (I highly recommend Ocean Basket) and escape the colder weather in other parts of Europe.
So there is no single “best” month.
It really depends on what kind of holiday you want. ☀️🌊