I guess, it is impossible to determine the best unit in general, gentlemen.
It is possible only in the considering of some unequivocal campaign, if we would look not on the unit's battle experience in general but on its battle merits. It is my strong opinion.
Even the famous 'L'Incomparable' 9e Régiment d'Infanterie Léger was really 'one of a kind' in the campaigns of 1799 and 1800 only - at Mincio and Marengo. In 1805, when 3/4 of the regimental strength were not those die-hard 'Italian' veterans but just regulars and conscripts, the 9th was an ordinary regiment already, and showed up no excellence . In the campaign of 1807, they even lost their Eagle - at Mohrungen, 25th January (from memoirs of Captain Reitzenstein: " ...My giants - the Yekaterinoslav Grenadiers - drove away those ragged, stunted French infantrymen just with their musket butts, and laughing chased them pretty long...").
Tirailleurs du Po - the former Expeditionary Piémontaise Battalion, and Tirailleurs Corses - they are one of so many fictional legends of LGA, it seems to me. With all my good will, I tried to find some remarkable facts of their battle history but in vain. They fought more or less successfully in 1805, 1806, 1809 campaigns like the absolute majority of LGA's infantry. They always brigaded with one of the best French regiments - 26e Léger - hereby all have explained.
The valiant Polish men of Légion de la Vistule: 1er, 2e and 3e Régiment d'Infantery and 1er Régiment de Lanciers - especially, they covered themselves with the unfailing glory in campaigns of 1808 and 1809, in Spain.
Zaragoza, Tudela, Talavera! It was their 'star hour'.
But the most admiration I feel after the battle merits of LGA's Infanterie Suisse: 1er, 2e, 3e, 4e Régiments.
1812. Russia. 1st and 2nd Battles of Polotsk - one of the most savage battles of the campaign. After that - the damned Berezina. Marshal St-Cyr: " I know the Swiss. ...The French are more impetuous in an advance, but if it comes to a defense, we can count of heroic calm and courage of the Swiss."
In Russia, the performance of the Swiss regiments was one of the finest by foreign troops during the whole of the Napoleonic wars. Napoleon himself mentioned the stalwart performance of the Swiss in a bulletin on the 1st November, 1812. The four regiments were awarded 34 Légion d'Honner for the 2nd Polotsk, and 62 for the Berezina.
Chest' imeyu!