With Fallout 4's new DLC, Far Harbor, comes new weapons and armor, and just before release Bethesda teased an armor set that would be the best available in the game. That set is the "Assault Marine Combat Armor."
As always with new stuff in Fallout 4, if you don't want to be spoiled on this one, turn away now because I'm about to spoil the crap out of how to find these pieces of sweet, sweet armor.
Alright, now that it's just us crabs, when you arrive in Far Harbor proper, similar to Skyrim's Dragonborn DLC and the Deathbrand armor one tracks down in Solstheim, the Assault Marine Combat armor is scattered across the foggy island in hidden "shipments".
No one is quite sure if there are prerequisite missions that you will need to have completed before searching for these pieces, as they are all static items in the overworld, but nonetheless, it is helpful to have finished the final memory sequence in the "Best Left Forgotten" quest before seeking them out. Then, you should have a miscellaneous quest to "Find All The Marine Combat Armor Shipments."
The First (Two) Piece(s):
Our first stop is slightly southeast-by southeast of the MS Azalea wreckage and east from the shoreline of the island.
Throw on your Hazmat Suit, pop some Rad-X, and let's go for a swim. When you see the mast of a wrecked ship, turn on your Pip-Boy's light and dive straight down. In a military chest will be both pieces of arm armor.
The Second (Two) Piece(s):
The second shipment you'll find is hiding due southwest of the Children of Atom shrine on the southwest corner of the island. Once again, get ready for a radioactive swim by prepping accordingly.
A ways out from the shore, you will find yet another wrecked ship, but this one you'll have to swim deep inside of to locate the shipment box. Inside, you'll find both bits of leg armor for the Marine Combat Suit.
The Third (And Final) Piece(s):
To the western side of the island, near Rayburn Point, is the third chest. Like the first two, protect yourself from radiation, and hop right into the water in southwesterly direction.
About sixty meters from the shore, you'll find a sunken shipment container. The military shipment inside contains the chest piece, not one but TWO helmets for the Marine Combat Armor AND a Wet Suit, which is the under-armor for this particular suit.
And there you have it, Wastelanders. Throw on your new duds and go murder some Mirelurks. You've earned it.