Ah geeze... first things that come to mind:

Mozart's "Magic Flute" Overture. (heck, "Marriage of Figaro" as well)
Beethoven's 5th Concerto (Emporer Concerto)

If you like solo-piano music:

Beethoven's Sonata "Waldstein" will blow your mind as well.
(Waldstein is one of his least known, better works)
Beethoven's Pathatique, Moonlight, etc.
Debussy's "Clair De Lune"
Lots of Rachmaninov Preludes.


All these pieces contain heavy themes that are either bombarded by variations, or very clear.

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