/u/BritainOpPlsNerf handled the notable cases but I'll handle the average day to day with exclusive respect to the French.

The most basic (ie: smallest) tactical body in the French military was the battalion. As in, the battalion of 840 men composed of a voltiguer (skirmisher) company, a grenadier (which I go over in some depth here for what made them so special) company and 4 fusilier (ie: line) company's.

Each company post 1808 would have a total of 140 men separated into two "sections". These sections would be 3 rows deep and have a corporal on each of the 4 corners; a lieutenant would be in charge of each section and placed dead center in the rear. A Company Captain would be placed at the right of the 1st section (traditionally also to the right of the 2nd) and would lead from there.

With 6 companies creating a Battalion however there needed to be more officers above the rank of Captain to maintain the force. In this you would have two Guide Général's on either side of the line or positioned at the front and back of the column to, as is clear in the name, guide the battalion and keep it from breaking apart. There would also be an adjutant and a sous-adjutant, equivalent to a Major. One would control 2 fusilier Company's and the voltiguers and the other the other 2 fusilier Company's and the Grenadiers (traditionally). In the case of column formation one adjutant would be placed at the front and one at the rear. The leader of the Battalion, the Chef de Bataillon would remain on horseback between the two adjutants. His rank can be loosely compared to that of a Colonel in our English system. It should also be noted that these rank titles, adjutant and Chef de Bataillon, are used in the French military to this day.

Now it's easy to say that the Chef de Bataillon, or the "Colonel", just sat back as his men walked in and did all the fighting; you could not be further from the truth in this regard. The base of the French model of fighting was the column attack; it was the foundation on which Napoleon built his tactics behind. Through combined arms the infantry organized in line would hold the enemy infantry in place, the cavalry would threaten the flanks and force the enemy to commit his reserves, and then the French reserves would strike the weakest point with their full weight along with the Grande Batterie. By the very nature of a column attack the entire Battalion is at direct risk as it is an inherently aggressive maneuver that puts everyone there rushing into enemy fire and eventually into meeting them in a melee. So in that regard if you were in the Battalion and were in a column attack you were in direct danger and could never 'loaf about' as the rest of the men did the fighting.

So I'd say for certain the highest ranking man on the 'front line' would be a rough equivalent to a Colonel, the Chef de Bataillon, ie: the Battalion commander. If you mean the literal frontline that would be the Company Captain who was literally on the first rank of the front line leading the men.

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John Elting, Swords Around a Throne: Napoleon's Grande Armée

If you are speaking of what we would now refer to as 'General Officer' ranks, then only in very isolated - but famous - incidents.

During the Siege of Toulon, a vigorous counterattack was personally led upon the French batteries by British General Charles O'Hara. During the course of his advance down overrun French trenches, he was taken in enfilade and in close quarters by a hodge-podge of French infantry and artillerymen, led by a young Artillery Officer by the name of Buonaparte. Allegedly O'Hara personally surrendered to this Officer from this rather compromising tactical position (Source: Chandler).

There is a more 'routine' example to be found in leaders of large-scale Cavalry corps, who would often begin a charge paces ahead of their men. As the distance closed and the danger grew exponentially, their gait would, almost imperceptibly, slacken so they could rejoin the safety of the main body, but it was not uncommon for a Cavalry General to find himself in the wild scrum, charge-and-repeat of a Napoleonic Cavalry melee. Joachim Murat famously (and contemptuously) led one of the largest Cavalry charges in history at the Battle of Eylau with nothing more than a riding crop in his hand.

Finally, one of the most famous cases of a General Officer leading personally would be Michel Ney during his rearguard actions in Russia. There is a famous period engraving of him holding a musket and personally fighting with the picked rearguard. While I'm sure there is much embellishment to how much danger he was in, there are too many sources from within the French camp to doubt he did so. Truly deserving of the honorific 'Bravest of the Brave.'

None of these men died while leading these personal actions, but such reckless bravery cost the French many excellent Division and Brigade commanders. Perhaps the most famous example is LaSalle, a rising star in the French cavalry who died leading furious charges at the head of his light horse division.

Hope that sheds some light! The fact that these are almost all near legendary should show you how very rare it was, and as for 'supreme commanders' doing so...well, Wellington summed it up nicely when he said that a General is meant to command, not swing a sword.