We will be in Athens in September 2025 and would like a private tour guide for two of the days. We would appreciate any recommendations from your experiences.
Since you didn't specify what kind of "tour" you're looking for:
Sites such as the Acropolis or other archaeological sites can be visited with a licensed guide. A (partial) list is available in the link below:
https://www.tourist-guides.gr/guides/list-of-licensed-tourist-guides/?bla=1338&q=Attica
Or you can check out these services that offer a selection of private tours:
https://www.athenswalkingtours.gr/athens-type/private-tours/index.1.html