Why is this the best armour? simple it has the highest total defence by a long way!
Umbral set = 1409 total defence | Titan set = 1006 total defence | Golem set = 1001 total defence (all before upgrades)
To see maxed out stats of this armour skip too 1:20 You must invest in points to rank 4 heavy armour and at least 30ish endurance to use this armour. In certain situations other amours may be more effective but overall this is the best most well rounded armour in the game
If you would like to read more on armour stats check out this reddit post for a spreadsheet on all the stats!