Who is Ron Weasley's favorite Quidditch player?

Nowhere in the series does he express any particular preference for one professional team over another... so anything we can suggest is likely no better than a guess. That said:

While he is in school, likely his own, Gryffindor.

After he graduates, likely the Holyhead Harpies, that being who Ginny is playing for.

I suspect his loyalty to Ron does not extend to his cheering on the underdog Chudley Cannons.