Obviously armour was on the decline by the 1600s. The standard form of a harness was the cuirassier's - flat breastplate, wide square pauldrons, long tassets, and a wide variety of helmets which ranged from old-school close helms to death's head burgonets to lobstertail pots. Most of them were of the basic, blackened, munitions-grade variety, but some commanders had spectacularly decorated cuirassier's harnesses, some of which remain to this day and some of which can be seen in portraits.
Frederik Hendrik, Prince of Orange:
Maurice of Nassau:
Louis XIII:
Dutch heavy cavalry armour:
French cuirassier's armour:
Armour of King Charles I of England:
Any others you think are worth sharing?