Not a terribly well-known composer, but his massive piano concerto is a ravishingly beautiful, high-romantic work (I like Hamelin under the label Hyperion), and his opera Doktor Faust is a complex masterpiece, stunning in its orchestral color (The only game in town is apparently Kent Nagano, although I don't like the Mephistopheles sung by Kim Begley: he is not devilish, merely bossy)
Talk about obscure! He's a hard composer to get to know. If you don't want the entire opera Doktor Faust, two sections (Sarabande and Cortege) have been extracted by the composer for concert use. There used to be a fine EMI recording, but I don't know if it's available any more. On CPO, there's an interesting disk of orchestra music which is well worth the investment. Very colorful and entertaining. If you're into chamber music, string quartets 1 & 2 are also on CPO and very enjoyable. A previously mentioned Piano Concerto is a must. I love music of this period, but Busoni is still hard to come by, and even then an acquired taste. Still, if you like it, try Enesco, too.
I guess the best know works are the Bach transcriptions (I have Horowitz playing some of these), but Marc-Andre Hamelin has recorded the Piano Concerto Op. 39 which is quite enjoyable. Hamelin seems to love finding obscure works for the virtuoso pianist to record and he doesn't disappoint.