Lance Armstrong is a great, but he isn't even the best cyclist ever, let alone the best sportsman... there was a really fair list done recently (with a full and open explanation as to how it was calculated) and Lance came in 11th, I think. There was a thread in 'race' about it.
It all depends how you see these things though - is being the greatest sportsman about pure sporting achievement or about the context too? This would include reaching the top agains the odds etc. Does it include the other traditional conceptions of what makes a sportsman apart from simply performance on the field? You know, all those 'amateur' values like fairness, character etc... W.G. Grace, for example, was the most horrendous cheat!