I dare to say that a XVI century knights in a full (and very expensive) plate armor on a horse which had its own armor too, could easily charge a line of XVIII century infantry and decimate the enemy.
I disagree, they had trouble enough just charging longbow men. Let me put it this way. While theoretically they could, the outcome is decided by so many factors both inside and outside of human control that what seems impossible at times may very well be possible.
Example the battle of Minden: Marshal Contades is reputed to have said bitterly after the battle: “I never thought to see a single line of infantry break through three lines of cavalry ranked in order of battle and tumble them to ruin.”
There's a whole list of failed and successful cav charges throughout history making assumptions like 'easy' hard to swallow.
I once read about a russian cavalryman who fired a pistol point blank at a french soldier - the ball failed to penetrate the skull and got stuck in the head wrappings. Granted it was very cold there and russians had very low quality powder at their disposal, but still you you have to remember that that musket balls are round and rather soft. A musket ball to penetrate a proper plate armor would need to be fired at really close range , so close actually that it would make the charging horse crash into the line.
The joys of dealing with black powder

misfires were also common and if the powder got wet from humidity or whatever reason you were in big trouble (as the musketeer).
Also, note that such units as Polish Winged Hussars had their own specials tactics of charging frontally infantry - they would disperse while approaching the line and in the last few meters they would close in the formation for the maximum effect.
Yes and they had very good results in a few battles against superior Russian forces, also and correct me if I am wrong this was probably before the bayonet was adopted for standard use.
As I said, 100 late middle ages knights would easily destroy 200 XVIII century infantrymen in a full frontal assault, but the problem was that 1 such knight with his horse and equipment would cost as much as 100 soldiers with muskets.
Yes its true that the cost would be enormous and even Napoleon was annoyed by the cost of his heavy cav's armor which I believe was apprx 8mm thick for the front and back plate, (I think the medieval plate armor was actually thinner (not sure)). In any case after much reading of various sources I can safely say that there is no certainty of victory in any case.
Napoleon's main use of heavy cavalry was to keep them in reserve and to trot them out only for decisive operations and mop ups. Occasionally however, when his situation grew desperate he did in fact trot them out for a frontal charge (as is the case in the battle of Eylau). At Eylau Napoleon sent Murat with 10,700 cavalry to charge the Russian center in order to buy himself some time. At one point Napoleon had to send in more cav to ensure the cavalry wouldn't become encircled. Some of the Russian infantry managed to withstand several charges before eventually succumbing or withdrawing. All in all the battle proved indecisive and the casualties to Murat's 10,000 strong cav force are estimated at over 1,500 although the exact numbers are unknown. It was by no means an easy victory, and yet it was the largest cavalry charge of its time.
Excerpt:
Napoleon saw that Murat would have to be cut free and ordered forward the
cavalry of the Guard. The guard cavalry smashed through everything, cutting a path for
Murat’s trapped cavalry to withdraw. The cost was heavy though. General Dahlmann,
Aide de Camp to Napoleon and previous commander of the Chasseurs of the Guard, was
killed. General Lepic, commander of the Guard Grenadiers, was wounded.45 Murat had
lost over 1,500 cavalry (either killed or wounded) in the assault. General D'Hautpoul,
who commanded the cuirassiers, was killed and General Grouchy was wounded.
Additionally, four regimental commanders were lost in what would become know as the
greatest cavalry charge of the Napoleonic wars.
http://www.jeux-histoire.fr/doc/MURPHY.pdf
Also, I like your Rejenorst for how much well you argument your opinions and back them up with facts. Its so rare actually that I must express my gratitude for your selfless pursuit of truth:D
Thank you
Also I leave you with an excerpt from a U.S. Bayonet Manual (1852 or 1854) which comments about 2 men who defeated 25 cavalry men with bayonets:
It will be proper to remark that any system
of fencing with the bayonet can, in service,
have its full and direct application
only when the men are isolated, or in very
open order; as, for instance, when employed
as skirmishers, in assaulting breaches, fieldworks,
or batteries, or when broken by cavalry,
etc. etc. When in the habitual formation,
as infantry of the line, the small interval
allowed each file, and the method of
action of masses, will prevent the possibility,
or necessity, of the employment of much
individual address; but even then, in the
shock of a charge, or when awaiting the attack
of cavalry, the men will surely be more
steady and composed, from the consciousness
of the fact that they can make good use of
their bayonets, and easily protect their persons
against everything but balls.
There is an instance on record of a French
grenadier, who, in the battle of Polotsk, defended
himself, with his bayonet, against the
simultaneous attack of eleven Russian grenadiers,
eight of whom he killed. In the battle
of Sanguessa, two soldiers of Abbe*'s division
defended themselves, with their bayonets,
against twenty-five Spanish cavalry, and,
after having inflicted several severe wounds,
rejoined their regiment without a scratch.
At that period there was little or no regular
instruction in the use of the bayonet.
http://www.7thmichigan.us/manuals/Bayonet.pdf