I'm not going to say what I really think, because, chances are, I'll be flamed for it.I'll go with Godowsky's sonata and Busoni's concerto.
Scriabin: 1st piano sonata, by far. This piece doesn't seem to be taken seriously even by most Scriabin fans. 7th sonata is my second choice. No one seems to play it, unless it's part of a compulsory "complete sonatas" set. Horowitz didn't. Sofronitsky didn't. Unlike the 1st, though, people seem to respect it.
The First Sonata isn't innovative at all
and I find it to be rather boring.
The Seventh isn't really underplayed, plenty of people have recorded it. However, it is horrendously difficult.
I don't agree. I don't have any other piece that sounds like it. As for being innovative, I don't think that's a requirement of great art, but it was innovative for Scriabin to use a funeral march as the final movement. Tchaikovsky did that shortly after in a symphony. Plus, I think the first movement is exceptional.To each their own. As with any Scriabin (and probably anything else), the performance can make or break the piece. I don't care for performances where the initial material is repeated. That optional repetition drains it. Hamelin, Taub, and Szidon all play the repeat. Ashkenazy, my choice for the piece, doesn't. The only problem with the 1st sonata is that the 2nd and 3rd movements can be rather bland if the pianist doesn't work wonders with them. This is typical of the earliest Scriabin attempts at large-form composition. The E flat minor is a much more obvious example of Scriabin's early difficulty with scale. Unlike the E flat minor, I think the F minor is a mature piece, although it requires an exceptional pianist to keep its from bogging down after the first, absolutely brilliant, movement.As for student-grade, that's my opinion of the E flat minor sonata. I especially loathe the clumsy opening material. The piece would have been much better without it. The simple melody that follows it is superior. The piece has some moments of brilliance, or near-brilliance, including in the possibly worse "Allegro Appassionata" condensation/revision.I know it's difficult, which is why most of the recordings out there aren't very good. But, I don't think it's played as frequently as it should be. Every recording I've seen of it has been part of a complete sonatas collection, even Glemser's, although he hasn't yet recorded them all, as far as I know.