I can't make this kind of list. Each composer has their own unique qualities that are important. 
So you're saying that you have no musical preferences? Yes, two composers might have qualities that are important, but some qualities can be considered more important than others, and in terms of personal preference, while it might be difficult to choose your favorite 10 and order them, I think you could narrow it down a bit. Are you saying J.S. Bach, who influenced countless composers, is no more important than any of his sons? Why, then, is J.S. extremely famous while his offspring's music is only known by some? Are you saying you would just as soon play Hanon as Chopin? Based on the amount of Chopin you've played, I would assume he would rank higher than Hanon on your list. "All composers are equal, just different" is a weak answer used when one is unwilling or incapable of being decisive.
Now, let's see if I can come up with 10. This will be a combination of how "objectively important" a composer is as well as my own taste.
1. J.S. Bach - Really set up a lot of things in the modern era of music. Also heavily influenced many of my favorite composers.
2. Beethoven - Not a big fan of his music, but he ushered in the Romantic era with his later piano sonatas.
3. Chopin - I'm a reluctant fan of his music, but also hugely important.
So that covers what are, in my opinion, the obvious 3 most important composers. As I've often heard said, the Well-tempered Clavier is the Old Testament, the Beethoven sonatas are the New Testament, and Chopin's Etudes are Revelation.
Now, here are some places I am less sure of.
4. Wagner - the Tristan chord is, by many, considered the start of modernism and the turn to atonality (although the chord itself is not all that unusual). I also like what I've heard of Wagner's opera.
5. Liszt - he established many of the modern standards for piano performance (although I've heard it said that Clara Schumann also helped with many of these) and was also a superb composer. I'm not sure about this part, but I've also heard Liszt's sonata's "single movement" form (although arguably 4 movements) was revolutionary.
6. Rachmaninoff - just for being an absolute genius.
7. Scriabin - cause what the hell was going on in that guy's mind? Like, really.
8. Sorabji - I'm not really familiar with many of his works, but again, what the hell, man? There's that point where you become so crazy people aren't sure if you're a genius or just crazy. He's well beyond that point, and I have no idea.
That's all I got. Eight. Many other composers I like, but don't really have a justification for putting them on here.