Obviously my first post was way too vague. I figured I'd make it so you can just pick the class, and find out for yourself. The game makes it pretty hard to fuck up a build if you have an idea in mind, and just read the talents.
Most monks dual wield their fists (especially with the boots that give unarmed damage), which reduces the relative penalty imposed by heavy armor, since it only affects recovery time, not actual attack time. It's also why anyone with a firearm should generally be wearing plate - it has no effect on either their attack speed, or reload time. Their recovery time is a very small fraction of their total attack time.
You said it yourself, they go down a lot in the early game. Plate makes sure that never happens. Most enemies that actually require you to use abilities do enough damage that you'll generate wounds faster than you can spend them anyway. Especially in the late areas, like Cragholdt you'll be taking a pounding anyway. The extra damage reduction you get from that talent will be extremely important when you will most definitely be spamming Flagellant's Path to get at those pesky backliners that would otherwise crush your entire party. Because he'll be behind the enemy's frontline, he needs to be as beefy and independent as possible. The WM bounty in the Searing Falls that rewards you with the Maegfolc Skull comes to mind. If you don't get someone behind their beefcakes you're going to get dismantled by Level 6 and 7 spells. Since the Monk will most likely be using the +Unarmed Damage shoes, Shod-in-Faith cheese is not available to keep them up longer, either.
If your Monk is a mix of damage dealer and off-tank, his job is typically to tank at least 2 enemies. With light armor against the difficult enemies, he will likely generate more wounds than he can spend, which is wasted resources. Also in light armor, his health dips too low, too quickly. If you're anything like me, you treat resting as a game within the game, and try to do as much content as possible without needing to do so.
https://forums.obsidian.net/topic/82367-class-build-the-juggernaut-a-heavy-armored-monk/ This is one of the more popular builds I've seen for Monk and it uses heavy armor. Like Excidium said, you can swap out armor as you need it, but I'm way too fucking lazy for that. It's also why I've avoided using Monk in general for my last few playthroughs, way too much babysitting his command bar for my tastes.
Personally, I prefer Fighter because their passive combined with Shod-in-Faith makes them virtually unkillable against everything but an Eyeless Fire Beam, or a Dragon wing smash, even while wielding Tidefall. They also have no need for Wound generation, and kick ass from the start of the fight. If you need to plug a hole, go for it. If you need to prioritize a backline target - Charge is available without needing 2 Wounds. High DR enemy? Sundering Blow is your go to. Wanna neutralize any threat that isn't immune? Knock Down does the trick and gives your casters enough time to get some big blows in. Large group of enemies about to make their way to your backline? Clear Away is your man. Fighters also have the highest base defenses in the game - so much so it more than makes up for Duality of Mortal Presence. Weapon specialization, and mastery gives them even more absurd amounts of damage. Soulbound weapons are affected by any weapon class talents, so it doesn't even matter if you pick Soldier - using Abydon's Hammer later on will still get the benefit. With resting bonuses, don't be surprised to see your Fighter sitting with 28 - 30 Might at all times if properly equipped.
I've built Hiravias tanky once. It makes his short range spells better (Crushing Wave is AWESOME from the front line), but he bears the risk of getting interrupted a lot, unfortunately. And when you need to cast spells, you usually need to chain cast them. This is when heavier armor starts getting ugly and noticeable on casters.
I've tried a frontline Wizard as well, but have always preferred a mix of range, and mid-line fighter. Concelhaut's Staff is a great level 1 spell to keep equipped on your Wizard for any situation. Slicken, and Chill Fog are probably the other must have level 1 spells. They also require lots of baby-sitting and spell casting EVERY fight to get their stats up to par. CItzal's Lance is level 5 (i think) and another great reach weapon to use. Spamming level 4 spells as a Wizard is pretty much the way to win.
Frontline Priest makes sense from the D&D perspective, but they have the worst base defenses in the game, and it shows. Durance's great Resolve alleviates this a bit, but his shitty Perception and Dexterity offset it just fine. Chanters are definitely better suited for either main tank while dumping dex, or as a gun-toting mid / back row guy, if you have other firearm wielders.
Rogue is the one class I've never played in a serious playthrough. Everything about them is just...boring. They get some cool debilitations, with massive damage potential from passives and lots of Full Attack abilities, but a lot of them are carbon copies with a different status effect. Blinding Strike, Hobbling Strike, Debilitating Strike, Sap are all pretty much the same damn thing. Every time I play, I mean to try and make one but they never fit in the party.
I made the PC a Tidefall-wielding melee Ranger once, and it was really effective to start because of the extra tank, accuracy and damage bonuses against anything my pet and I targeted, and from a RP perspective (which I don't do), but became pretty boring at higher levels because the only talents that aren't ranged-based apply to the pet instead of you.
I'm a 3 front-liner kinda guy. In tighter spaces it can cause some problems, but it allows for flexibility of formations and makes for a HELL of a weapon when I only need 2 guys to block my back row and can send my main tank in with Ectophysic Echo on, and watch as he mangles everything with a Ghostbusters Proton Pack on his back. Works wonders with Charge, Flagellant's Path, or Dragon Leap as well. I also usually put 5 or even all 6 guys in plate because why the fuck not. NEVER need to rest on Hard. I'm also too lazy to be bothered with buffing and shit for PotD. Maybe one day.