When was the RAF at its most capable?
November 1918. No question, no contest in my mind. At this time the RAF was the largest airforce in the world, equipped with the most modern equipment, bar the Fokker DVII, several of it's types serving into the 30s. Each aircraft was designed for a specific role on and beyond the battlefield and, most importantly, it was all battle proven.
Also, apart from from some French engines, all the equipment was manufactured in Britain.
Fascinating question.