I would be interested in how people here rank the CAS systems for each major country in WW2 - British, US (navy vs. army), Soviet, German, Japanese (navy vs. army), Italian, Finnish, French whatever. Maybe for early vs. late war (with some participants not around in the later period).
The big question and probably the most controversial in here, is how effective was the Soviet CAS. The Sturmovik, the Pe-2, the fighters armed with rockets. A lot of people say they weren't very... but somebody wrecked a lot of German tanks on the Eastern Front.
By 1943 the Soviet CAS doctrine was pretty well advanced. I would say the British had the best system overall, but the IL2 was by far the best CAS aircraft out there.