I think that sheer athleticism and total dominance of one sport there really cannot be much to equal Eddy Merckx.
He won five Tour De France, one of which he had to do without the support of his team following an argument, to win a tour under such circumstances is unthinkable, no one else has won the tour without team support, and it is extremely unlikely that it is actually possible.
Add to this his career record of 34 Tour De France stage wins, plus all the other races he won, and he held the world hour record - which could only be broken some 12 years later at altitude, on a far more technically advanced machine(which is also now banned due to limitations on the size of the back wheel)
Had Merckx been riding such a machine at the same altitude with the same amount of preparation time one can only speculate what he would have achieved.
Add in his 6 day track event wins, something that few road riders now make much of a priority, it is the breadth of his achievements in cycling, beating specialists in fairly narrow fields.
First man ever to win the Tour De France, The Giro D’Italia, and the World Road Race Championship titles in one year.
Look at this for a sporting career,
http://townsleyb.members.beeb.net/procycle/merckxe.htm
As for one hard man, he was attacked by French spectator in the 1975 Tour De France on one horrible climb which affected him for some days afterwards, he fell a couple of days later and fractured a cheekbone, and due to drug control was not allowed any painkillers, anyone would have forgiven his retiring the race, he had little chance of winning, but he continued and finished second.