Heavy Armor
OK, this was going to be just a single entry but it kept growing … so here we all are.
Heavy armor is the more obtainable pinnacle of personal protection compared to the exotic stuff. This is usually very high quality steel plate reinforced with flexible mail (we call it chain mail nowadays, which riles the historian class, but honestly, most people don’t care, call it what you want), worn over a good deal of padding. This level of armor offers incredible personal protection at the cost of some mobility - but it doesn’t just make you slow. Well crafted plate imposes some restrictions on your movement - for example, you can’t turn your head in the full range of motion you are used to, or your shoulder rotation might be restricted - but for wearers who are accustomed to the limitations placed on them, something akin to full mobility can be achieved; there are stories of knights (who trained quite a lot) being able to leap into the saddle of a warhorse in full harness. So don’t assume the guy in the plate and mail is a slow armor turtle. He’s tough and can be quick in some motions. You can run just fine in plate - but it’s really, really noisy (unless you’ve taken measures to try to keep it quiet), it takes about twice as long to accelerate, and it also takes twice as long to slow down. (Also, kindly meant piece of advice- don’t step in front of a running man in plate expecting to check them to a halt with your shield. A normal size guy is probably 250 pounds plus of meat and steel at a full run … and he can’t stop … you’re gonna get flattened). Keep in mind that armor is very insulating and is always worn over quite a lot of padding, so it can be really, really warm. You can get out of breath and overheated pretty swiftly.
The most common full plate is what we think of from Agincourt or the Wars of the Roses; it is quite good protection that covers just about every exposed point of the wearer. This level of armor will turn a lot of weapon hits; including most arrows (plate stopped a good amount of early gunpowder weapons too, for a while). Swords can harm somebody in plate - you’ll need to strike a joint or weak point - but this level of protection is why we get a lot of the anti-armor melee weapons like warhammers and halberds. While fully armored fighters like this are tough - and much, much more dangerous when on a horse - they dislike fighting in bad footing, will sometimes struggle with visibility, and they do get tired faster than a combatant not so well protected. Something to keep in mind when writing your knights in shining armor.
It is worth noting that armor in this category is usually carefully fitted to the wearer, you could probably borrow your friend’s armor if they were close to your size and build, but you might need to fiddle with a strap or three. Wearing armor of someone with a dissimilar build would be highly uncomfortable and might fail completely in some cases.
There are lighter options; wearing only part of the harness (such as not bothering with the leg armor in some cases), or wearing a lighter assembly of plates in key areas; like breast and back combined with partial arm and leg armor. Even as gunpowder weapons got better and full armor was abandoned, the breastplate was retained for a long, long time.
A word about helmets - we often see people going without helmets in movies, because we really want to see the actor’s faces. In action, the helmet is the single best piece of armor that nearly everyone wears, because protecting your head is a really good idea. Write what works for your story, but a brain bucket is never a bad idea.