My own personal top 10 with personal/half-assed reasonings. Not too familiar with anything past 1750, but ill try to include a few.
1.Antonio Vivaldi: No, seriously. Greatest melodist of all time, early adopter of the 3 movement ritornello concerto which was the basis for many forms, not just concerti. He manages to express deep musical emotions through harmonies that would leave other composers unable to write anything other than student excercises. He "says" things often through a simple, ingenious ritournello theme. And often manages to say as many "things" in that short theme that would take other composers an entire movement.
2.Jean-Baptiste Lully: No Lully? no Rameau. Also very grand and majestic music, combined with great emitonal depth and expressiveness in his operas. Also because french opera is superior to italian (in terms of form)
3.Johann Sebastian Bach: His instrumental music is absolutely inhuman in its technical perfection, which is also his greatest weakness, but thats another point for another thread...
4.Giovann Pierluigi da Palestrina: Greatest contrapunctist of all time. Austere, "clean" All around untouchable in terms of quality, because his music embodies the fundamental qualities of western music, and nothing more, perfectly.
5.Heinrich Schütz: Greatest composer of german sacred music of all time. I see him as a lutheran, spiritual succesor to Palestrina.
6.Schubert: Token romantic composer on this list which is by now choking on wig powder and stockings. Seriously though, he was great, for many of the same reasons i like Vivaldi (melodic genious)
7.Luigi Boccherini: It was either him or Haydn for my classical era choice (Mozart has been played everywhere so much it has become annoying to listen to, otherwise he would be in this spot.) Boccherini had most of the same qualities as Mozart and Haydn, but has a more easy going (some would say superficial) and light style, sort of vivaldian in this.
8.John Dowland: Screw Mccartney/Lennon. Dowland was the greatest English songwriter of all time.
9.Claudio Monteverdi: Revolutionary in every sense of the word. Listen to his madrigals, then listen to those of his contemporaries, the difference is amazing in terms of harmony/counterpoint.
10.Guillaume Dufay: Ending with the start. One of the first great composers, full stop. Late medieval/early renaissance composer who accomplished quite a lot with a fairly limited contrapunctal/harmonic palette. His (and Binchois's) music almost doesn't sound western, wich gives it a nice exotic touch.