Like ElViejo said, blasphemy is dishonouring God in one's speech and Jesus was accused of blasphemy. Blasphemy, in essence, is a man-made sin, for me.
We do not know what God wants from us, so we cannot know if he will be angry if one dishonours him in His speech and what He considers as 'dishonour' in itself anyway. Furthermore, as God is not human, we cannot assign to him human feelings as dishonour, anger, disapproval. If we do not know God, as such, how can we say He feels dishonoured when spoken about in a certain way? If God is love, then surely He does not get excited about petty things as 'dishonour'.
If Jesus was accused of blasphemy, how can that be if He was the son of God? Did the Jews not have the right God to believe in? That cannot be right, because they were created by that same God. For a time the Catholic Church has claimed to be the only true religion, but on that note, they have come back.
To me, blasphemy, is more an utterance of over-piety of the accuser than it is a real sin.
Killing is a sin, stealing is a sin,... but those things are all universally wrong anyway and are disapproved of in all societies. Blasphemy has no definition and no moral ground apart from a human non-universal one. And thus for me it cannot be a sin, far less the greatest sin. It is man-made 'sin' out of fear, which should not be felt for God if God is love.
This was an interesting question...
Now, don't think I am over-pious as well... I sometimes get scorned on another forum for religious assertions about certain works. Religion is not very popular and somehow unreligious people can't stand religious people, I have the impression. I have no problem in this scientific world and I embrace everything and do not se God in every plant and every little cobble stone as some Christians do, but I do have certain beliefs which I do not hide.

