Best,ML
The Busoni Concerto takes the cake I think. Moreso than Rach 3, which is just a little runt compared to it. There are numerous 20thcentury/contemporary pieces that would qualify, but my brain hurts too much right now to think of one.
The "skyscraper chords" (as Raymond Lewenthal termed them) in Alkan's op 33 should surely be mentioned.