Wellington said that as a unit the French heavy cavalry were better in large numbers, while the British heavy cavalry were better individually. At Waterloo, what kind of equipment did the British heavy cavalry have that made people designate them as 'heavy'? Isn't armor useless in the gunpowder era?

More can always be said on the matter, so if anyone wants to offer their rundown on the features of Napoleonic-era heavy cavalry, go right ahead! For the meantime, there are some previous posts that I recommend to your attention on just this matter:

u/Bacarruda has an overview on the various types of cavalry of the period;
u/waldo672 has an overview of their own of the cavalry types as well;
and on your question of armour, u/AncientHistory examines the performance of cuirassier armour of the period.