Now, in a massive burst of procrastination, I read the entire thread, because I do think it's very interesting what people think, and in a way I agree that there shouldn't be much discussion beyond Bach / Mozart / Beethoven, mainly in terms of influence. However I would seriously add two composers to this list, both of whom I believe, allowing for their influences either to mature just as those of B/M/B, or become as well known, should be considered the 'greatest' composers.
1. Monteverdi - I do find it quite astonishing nobody has mentioned him before. There's no point talking about how many operas or large-scale religious works 'great' composers have written: Monteverdi practically
invented dramatic music (madrigals and his 19 operas), unprepared dissonance, large-scale choral works (Vespers of 1610),
instrumental writing itself as distinct from vocal writing; was probably the
most innovative and divergent composer from the prevalent style of his time out of B/M/B; was the first composer ever to specify particular instruments for parts, and particular playing techniques (tremolo and pizzicato for strings in his 8th book of madrigals); and basically single-handedly laid the way for the Baroque. When you consider where he came from, where he ended up, what musical concepts were literally non-existent before him, and also (of course) his immense talent in writing exciting, beautiful, colourful music, I think he absolutely gets my vote.
2. Debussy - but this is an area I'm less knowledgable about. And I have to take the dog for a walk.

So I'll talk about him another day. (I considered Fauré as the 'root' of impressionism, but Debussy's output and influence prior to that is definitely 'greater').
I know this is just a 3-year-old poll, and I'm not in any way angry with it lol. But it is interesting and I do think Monteverdi deserves his due.