But it is quite an interesting question, so let's take a poke at it. The word "great" can cause a lot of conniptions these days because it implies, shudder, inequality and even, shudder again, judgement about quality. Now I don't think we should be too afraid of making judgements, that is in fact what our minds are for, among other things. The only thing that should worry us is that we make well-founded judgements.
In the case of Mozart, this has already been taken care of for us by History. Over the last two hundred years or so audiences and musicians have clearly expressed a great appreciation for the music of Mozart, to the extent that I doubt that too many would disagree. In fact, I don't recall ever having read a serious critique of Mozart saying he was not a great composer. Hmm, that sounds like an interesting challenge... But no, let's not! Instead, let's try and pick out some of the reasons, general and specific, why Mozart really stands out as a composer.
He is one of the great opera composers with spectacularly successful examples like Don Giovanni, Così fan tutte, The Magic Flute, The Marriage of Figaro and others. He is one of the great concerto composers with the string of absolutely lovely and elegant concertos he wrote in Vienna in the 1780s. Here is an example:
He is one of the great symphony composers whose later symphonies are some of the most popular ever written. Here is his last, the Symphony No. 41:
He is one of the great chamber music composers who wrote some pretty good string quartets, some superb quintets and the first great string trio:
He is, in fact, the only composer who excelled equally at both opera and instrumental music.
So that's the general. But what is it specifically about Mozart's music that distinguishes it from the music of so many of his humdrum contemporaries? There is no simple answer to that question. I could say that Mozart has a charm, a clarity, a balance and an economy of means that the others lack, but that really doesn't tell you anything, does it?
The answer is going to involve looking at a piece or two in some detail. One person that has done this is Charles Rosen in two superb books: The Classical Style and Sonata Forms. In them he spends hundreds of pages talking about quite a number of pieces by Mozart in some detail. I think I might try to do something similar, but that will be in another post. In the meantime, enjoy the pieces I posted above!