An anonymous commenter got a bit heated a while ago when I dared to say that there hasn't been a great composer since Stravinsky. I think possibly his/her problem was confusing great music with great composers. Let me explain.
I think it is entirely possible for a good, but not great, composer to produce a great piece. But being a truly great composer requires more - a whole swathe of great music and a shift in the nature of music itself from that of his/her* contemporaries. (*Political correctness, I suspect. I don't think there have been any great female composers yet.)
Just to stress the lack of need to be a great composer to produce great music, my absolute favourite piece is by someone I couldn't regard as a great composer, as are several more of my top ten.
Oddly, I was introduced to my favourite piece by a game. Many moons ago I used to review games for various VNU publications, and one game (I can't remember its name) started with a bit of video giving backstory of how the human race was forced to leave the Earth. In the background, as the ships departed the dying planet, was the most evocative, heart-tugging piece of music I had ever heard. It turned out to be Barber's Agnus Dei, the vocal arrangement of his Adagio for Strings, which turned out even better than the original.
Another example of a composer even Anonymous couldn't regard as great coming up with a stunning piece, is down to one Robert Lucas de Pearsall (no, really) an early Victorian composer who wrote the very impressive 8 part arrangement of In Dulce Jubilo in Carols for Choirs. His masterpiece is even more surprising than Barber's adaptation, as it's essentially pastiche. Yet Pearsall's Lay a Garland - in essence a cod slow madrigal well after its time - is simply wonderful.
And finally, just to rub the point home, not only a piece I like, but the UK's favourite piece of classical music according to Classic FM, and yet by a composer not generally regarded as a great. This is Ralph Vaughan Williams. While he has more excellent compositions to his name than Barber or Pearsall, he still doesn't quite make it into the ranks of the mighty. Yet who can resist the eloquent summeryness of The Lark Ascending? Not I, for one.