When closely managed by the boss, most French marshals were competent. But outside Napoleon's direct supervision few of them really distinguished themselves. It seems that their performance dropped off considerably when their responsibilities increased.
I can't really name one who did great alone. Perhaps Massena or Lannes? Davout as far as I know did very well with one corps at Auerstadt, but never commanded his own campaign.
It seemed odd to me that Napoleon, an incredibly skilled general with complete control over promotions, wound up selecting mediocrities like MacDonald to command large forces.
How could Napoleon get a better crop of marshals under him?
Some potential ideas:
1. Better education for his existing marshals? Can't recall where I read it but apparently Napoleon didn't make any serious effort to train up his marshals or impart his thinking on them. Either a great oversight on his part, or paranoia about training someone to fulfill his role?
2. Exercises in independent command? OTL Napoleon selected his competent corps and division commanders. But they rarely got the chance to improvise and think for themselves, as Napoleon was their commander in chief.
Historically mediocrities can be removed through large scale exercises (e.g. Louisiana maneuevers of US army before WW2). Have Napoleon observe a marshal moving different corps around and evaluate them as independent leaders, not subordinates. But it feels a bit too much of a 20th century idea to me. Did any nation do something similar before at this time?
3. Using other generals who made their name without Napoleon? Jean Moreau comes to mind, as he defeated the Austrians in 1800 while Napoleon was doing his own thing in Italy. Of course, some POD will be needed to stop his exile. Those with experience as independent army commanders will be very valuable after 1808.
4. A different command culture/mindset for Napoleon? Could Napoleon have accommodated more initiative for subordinates ala Moltke? IMO, quite a stretch. Those ideas came many decades after Waterloo and was not in the nature of how he fought. Perhaps a staff and education system similar to what the Prussians implemented post Jena? But I doubt he would take lessons from the Prussians who he had trounced so badly, not until it would be too late.
Theoretically speaking if such a system were to be implemented, the Marshals would be selected/experienced better as independent commanders. Still, I think this is the least likely, unless a POD results in a major defeat for Napoleon by an army using such a system, while giving time for him to implement it.