I have seen it written, and with good reason, that the year 1810 was considered the zenith of the Empire.
When Napoleon left Spain in January 1809 because Austria was once again causing trouble, the Spanish had been repeatedly defeated and Moore's army was being pursued to Corunna from which it embarked.
The Army of Germany was built around the 90,000 veteran troops that Napoleon left in central Europe after Tilsit, commanded by Davout. These veterans were Davout's III Corps, St Hilaire's veteran division, and the heavy cavalry.
The new II and IV Corps were formed for the new campaign and the Imperial Guard was en route from Spain, as were some veteran commanders, such as Lannes.
The new conscripts that were sent to the Army of Germany definitely proved themselves, especially at Essling, and Napoleon considered the first half of the campaign in the fighting south of Ratisbon some of the finest of his career.
The performance of the II and IV Corps at Essling, fighting outnumbered for two days, speaks volumes for both the troops and the quality of leadership. And there were Confederation of the Rhine troops who fought alongside their allies there. They were definitely not third rate.
I wouldn't consider the Confederation of the Rhine troops from Bavaria, Hesse-Darmstadt, Baden and Wurttenberg 'third rate allies.' In point of fact they were excellent troops who performed excellently during the campaign. There were exceptions, such as the Saxons, with the exception of their cavalry, but in 1809 they were the exception, not the rule, in the Confederation of the Rhine.
The Army of Germany became an excellent army through the experience of combat and further training after Znaim during the truce that led to the end of the war.
Excellent sources on the subject are Saski's three-volume study and John Gill's With Eagles to Glory, which is a study of the Confederation of the Rhine troops in 1809.
I would therefore agree that the year 1810 was the zenith of the Empire.