Hard to say, but for me it was between 1973 (Recording BAND ON THE RUN) 'til 1976(Performing with a very well rehearshed Rock-Band during the WINGS OVER EUROPE / AMERICA tour).
You may say "WINGS AT THE SPEED OF SOUND is mostly terrible", and I would have to admit, you're right, but listening to the triple LP / double CD WINGS OVER AMERICA , recorded live in the same year, you'll understand, what I talked about above.