Lets have some controversy shall we?
I believe that the concept of the Space Marine chapter is best reflected in the real world by one organization, The SS.
I come to this conclusion based not just on a military perspective, but also their political position and attitude within the Imperium.
They are a self contained state within a state, they are a completely independent military force, their recruitment practices are extreme, they are fanatically devoted to the Emperor, they are genetically engineered to be Superior human beings (the ****s could only dream of that) . SS Divisions were moved from one front to the other to bolster flagging regular forces and were used at the spearhead of most major assaults, without them the Eastern Front would have collapsed on several occasions. The SS also were used as the Big Stick to keep the rest of Germany's forces in line, the Party's private army, see any similarities?
Are they tough? Yep. Are they the best troops in the Imperium? You bet. Does the Imperium talk them up with propaganda to make the population believe they are superhuman gods of war? Damm right! Its in their interests to make the people fear them.
From a purely military perspective you can make several comparisons with real world units, but the one that fits the best is probably the U.S. Marine Corps. A self contained all arms force of elite soldiers that is moved around by a huge Navy.
It is the fact he can transcend traditional tactics.
Yes, but tactics are still needed, just different ones. Superior infantry forces throughout history have never been about equipment, its about training and small unit tactics. This was true of the Roman Legions and will always be as long as man walks on the battlefield. The Roman Legionary in his lōrīca segmentāta must have had much the same effect on his barbarian neighbor as the Space Marine does on his opponent's but he still had to fight as a unit and use tactics peculiar to the Roman army for it to work. When ever those tactics weren't used or broke down the invincible Legions got slaughtered.