Consider this:
Take one single marine, unarmed and unarmoured, and put him in a fighting pit.
Keeping adding human men to the fighting pit, equally armed and armoured.
IMO Space Marine continues to fight and win, endlessly, untill his body dies from lack of nutrition - and considering SM physiology, it's a long time.
SM's are so much better than human warriors in every way that the tabletop doesn't really do it justice. Then again, an SM army would be unplayable if it id - mainly because the 20 or so tactical marines and hell knows what else you would deploy would wipe out bazillions of whatever your enemy pours out, other than CSM, etc.
Thinking about it logically:
A basic SM is likely to be a far better shot than even the most experience, skillful, and lucky human sniper, with any weapon.
A basic SM (not even a particularly hard one) is going to be orders of magnitude tougher, stronger, faster, and more skillful in unarmed combat than even the most badass human martial artist.
With a close combat weapon, the above applies.
A SM is profficient, if not a master, in the use of every weapon that a humanoid of the imperium will encounter in an armoury. If they are not a total master, and they don't prefer it, you can bet that they still practice with it anyway - they just practice MORE with the weapons that they DO prefer.
Space marine armour and weaponry is (if you are a reader/player of Dark Heresy, this should be especially apparent) relatively better than human equipment in the same way their physiques are. A space marine boltgun, for example, is WAY more badass than a boltgun carried by some random human knobber. The same applies to astartes pwer armour - all SM equipment is about as good as it gets.
I havn't even started on the physiology yet, as it would go on for ages. With regard to that, let's just say that they aren't fucking about. Muscularly, they don't really compare. With no enhancement (from powered armour, and such), they are described as being easily able to punch the head clean off a man's shoulders. It wouldn't surprise me if they could quite easily kick through brick walls and such, without armour. At least, the literature from the "inquisitor" game depicts them that way, and this is certainly the only game other than DH that scales the marine power/ability to what it should be: One SM can basically take out entire parties of other characters without even taking much damage. Including inquisitors, psykers, bodyguards, servitors, blah blah - someone brings a marine to an inquisitor match and the complaints start rolling out, mainly because the player without the marine gonna lose.
Why he gonna lose?
'cus, among other things, the marine knows exactly where all of the other players are from turn 1: He has an auspex. If they are immune to the detection from that, he has supernaturally good hearing, so knows where you are. If you are silent, as soon as you come into visual range (which, for him, is the full length of whatever game board you are using, considering not many people have a 20ft game board to mess with, to make that untrue), he sees you (this fucker does not fail tests). He has no fear. His armour takes nukes with joy. He heals, per turn, more damage than most characters not packing a multi-melta can deal. Without any weapons, his strength bonus deals, on average rolls, more damage than a human wielded power weapon (his fists are death, basically). To a human, his boltgun is a nuke. One shot = DEAD. Forget this 40k S4 ap5 bullshit. Bolter in a game of "true" power scale = WIN. Oh yeah, he does OCCASSIONALLY miss, but he just fired, like, 4 shots from his bolter. So with a hit rate, at lowest, of like 75%, you just got hit. You're dead. If you got into CC with him, well... that's nice for you... he adds his power-weapon-magnitude strength bonus to his... power weapon attack... Yeah, if you weren't dead before, you certainly are now. He didn't even bother dirtying his sword - he just headbutted you and caved in your face, while mumbling something about the Emperor's wrath. You're an assassin, you say? So you were silent, immune to detection, and slipped by unseen... to poison him with some ridiculously useless and expensive weapon... ok... Shame he's immune to that, too. Funnily enough, he also has rules stating that he can then bite you as an attack, and deliver the poison his body was subjected to BACK into you, instead. Nice. Fucked by your own plan. If that weren't cool enough, one of his attacks is being able to spit acid. FUCK your armour, sunshine. He has a knife too, somewhere, I think. If he bothered to take grenades, it's worth pointing out that after a small calculation considering his strength bonus, he can throw a grenade a distance over 100inches, quite easily. Got a big game board, have you? 'cus your character that is not a marine is struggling to ge their 'nade further than 20 inches. Imagine what the guy does with his knife, should he decide to throw it. So, you can't run, because he's way faster. You can't fight him, because you're gonna die. Hide? Well, that's fine... untill he casually walks through the reinforced wall right next to you, robocop style, and pops your head like a zit, between his thumb and index finger. Strength of like 250% + power armour = wall fail.
The only character harder/more ridiculous than the space marine is an officio assassinorum assassin. If the marine can take, say, at least 5 other characters without breaking a sweat, then the "official" assassin can take at least 2 or 3 marines without crying about it. Their rules were off the hook, and, frankly, taking quite a large amount of piss.
I would definitely believe that in the right circumstances, a marine would hold off hundreds of humans.
