Dragons beat all.
But if plate > everything, how does anybody get killed? It seems like it would reduce a battle to just knocking each other about until everyone gets tired and decides to go home. Why would knights even bother with swords?
Anders' post is good enough. It's my understanding that it's more iron chainmail trumps iron plate, while advances in steel metallurgy gradually made steel plate the better option.
Full plate was also generally intended for mounted use
Erm, I'm not sure I've ever even heard of "iron" plate armor - it wasn't really an iron age thing at all. Plate armor didn't really appear until the 14th century, well into the medieval era, and by then decent steel was totally a thing. Before then, armor mostly consisted of mail.
Easy: Plate was mad expensive. Not just in terms of materials and craftmanship; they were very complex things that basically had to be tailor made to fit the owner. You couldn't possibly outfit a whole army - especially not when everyone was expected to provide their own equipment. They were mostly for the knights and nobles, and those guys could still be overwhelmed by a lot of dudes in inferior armor. So, if the rest of your army lost, it didn't really matter what you were wearing.
Easy: Plate was mad expensive. Not just in terms of materials and craftmanship; they were very complex things that basically had to be tailor made to fit the owner. You couldn't possibly outfit a whole army - especially not when everyone was expected to provide their own equipment.
Chain is a lot easier to make than plate (plate of a quality good enough to be a help rather than a hindrance.) Chain is also far more flexible than plate, so it hampers your movement far less.
I'm not sure that's true. Chain is flexible, in that all its weight hangs and shifts on you; is it really easier to move in than plates that fit tight around the different parts of your body?
Mail was about as heavy as plate was. When you're sinking to the bottom, getting out of your armor is impossible. Even mail was strapped to your person with belts (else it would shift around too much). Also, strapping on mail does take less time but how often do you get ambushed when you're out on a hunt? Because other than hunting, war or traveling, a knight doesn't leave his keep often. And when you're traveling or going to war, there's a reasonable expectation there will be trouble. So I'd put on my plate then.
Also, the most dangerous enemy for someone in plate is a misericorde. These knives were carried around to finish off fallen knights (both friendly and hostile - only the heavily wounded of course because you can ransom/heal the others). A misericorde looks like a letter-opener. It's a very slim knife made for one purpose only - stabbing through gaps in armor/eye-slits etc. Needless to say, knights weren't very fond of them.
I imagine that a direct hit with one of these: ((military flail)) Would pierce any kind of armor and kill your enemy, especially if you hit in the head!!
Also, I was thinking that if you throw Magic into the battle scene then the metal (plate armor, chainmail or whatever) would attract the lightning bolts straight towards you, with terrible consequences...