Missing Fallout 4's most powerful armour is pretty easy, given that it's inside of an unmarked building. Here's how you can find it.
What you're looking at above is the X-01 Power Armour. This puppy packs a ton of damage resistance, as you can see here. While you can find pieces of this armour out in a few different places out in the wild, there's a guaranteed way to find a full suit.
To gain access to the full X-01 suit, you'll need to head to a place called "Custom House Tower," which is a bit east of Goodneighbor.
You'll know you're in front of it when you see this building:
Once there, head to the left. You're looking for a green pastel building, which has a sign that reads "35 Court" outside. Here's a quick video guide, which should help you spot it/that you can use to make sure you're going the right way:
That 35 Court building is the unmarked building housing your Power Armour. It's worth noting that the suit of armour waiting inside actually scales to your level, so you can't just get the X-01 right away. You have to be in the upper 20's, levels-wise, to get the X-01 to spawn inside. I went in at level 27, if that helps.
In any case, the road to 35 Court might throw a few raiders at you along the way, but it's never a particularly difficult fight. Kill whatever enemies stand in your way, and head inside of the green building. There will probably be a few enemies inside of 35 Court itself too, as well as a few turrets, but it's nothing you can't handle. Once you've swept the area, take the elevator on the first floor up.
On this new floor, head outside. Here is where things get tricky: an alarm will go off, and both an Assaultron and a Sentry Bot will come flying out. Prepare yourself for this, because it's kind of a tough fight. I'd recommend coming decked in Power Armour to help, but you're here to collect Power Armour, so swinging that might be a little difficult. Just bring enough Stimpaks and some Pulse grenades, and you should be fine.
Once you destroy both the robots, head into their holding chambers. You should find a button in each one. Press it, and the station at the center of the roof should open up. Inside is the X-01. Congrats! You now have the best armour in the game. A full suit of it, to boot.
It gets better, of course. You can mod every piece of the X-01 to be stronger, or to suit your particular playstyle. The entire suit starts out as a Mark I, but I altered it to have a higher mark/damage resistance, as well as bonuses for VATs. Despite hoarding a good number of resources, I could only mod part of the suit -- so you'll probably end up going on a scavenger hunt if you want the best version of the X-01. But, it will be worth it. It literally does not get better than this in Fallout 4, so.
And if you're hurting on fusion cores to power the suit, make sure to check out our tips here. Have fun!