If we're talking about the impact or influence of a musician/composer, you might as well leave Mozart off the list entirely. It's kind of a bizarre historical happenstance that he manages to be one of the most popular composers today when he was largely a footnote during his own, and subsequent, generations.
I mentioned earlier that he was roundly considered a has-been during the latter portion of his life; now, that wasn't because of some conspiracy contrived by Salieri or whatever other bullshit has been fed to you by pop culture. The reason he died poor was because his contribution to the genre was actually quite minimal. What he did contribute was a melding of lyrical Italian operatic melodies into the instrumental Mannheim and Berlin schools (Style Galant/empfindsamer Stil). I mean, Haydn, who was 24 years older than Mozart, was still evolving his style by the time Mozart had already grown stale and keeled over.
If you examine the following generation -- Beethoven pretty much single-handedly dragged the medium into the Romantic period (musical periods tended to trail their artistic and literary counterparts by roughly 30 years due to the complexities of translating a movement into a completely interpretive medium [if we're talking about instrumental music]) -- Haydn had a far greater impact on the development of western music than Mozart, and it's not even close. This is kind of similar as to how we now revere J.S. Bach, but completely ignore his kids that were the innovators of what became the "classical" period. The only reason we remember J.S. so fondly is because Felix Mendelssohn started a Bach revival of sorts by conducting the St. Mathew's Passion for the first time in, likely, damn near a century -- thus starting the first wave of public appreciation of "antique" or "classical" music.
Now, this is not to say that Mozart was a hack or anything, I mean the Kyrie of his Requiem is a double-fugue in the form of a sonata...that is some fucking brilliant shit going on there. But, he was never a Lennon or McCartney in his own time -- more like a Kenny Wayne Shepherd or Jonny Lang: kids who were phenomenal musicians at a young age and got tons of accolades, but don't really have much to contribute as adults. With most composers you can identify a progressive early-middle-mature stylistic progression in their work, while Mozart's progression was basically "child prodigy with no particular style" - "jazz fusionist of the late 18th century" - "early-middle-aged dude that hasn't moved past his jazz fusion days." Mozart was incredibly famous as a child, had a decent early career and did some innovation, then got locked into his style and evolved very little for the last decade plus of his life while other composers kept moving forward.
Again, it isn't that I don't thoroughly enjoy some his work, but him always being held up as some sort of paragon of musicianship/innovation is just kind of ignorant of the historical reality. Unless, of course, if you're suggesting that McCartney is incredibly overrated by popular culture just as Mozart is; I guess you could argue that if it's what you're going for.
