Let us not forget our friend Jonathan Powell. He has championed lots of "impossible pieces" by Sorabji and Finnissy, but has also played some more "traditional" repertoire like Chopin and Medtner. Very well, I might add.
Seconded!
I've always strongly appreciated pianists who are willing to take on unperformed/unrecorded repertoire and Powell takes that to a new level without losing sight of the more standard repertoire. Other pianists who have opened my ears to underperformed repertoire:
Marie-Catherine Girod (Dutilleux's sonata, Auric's sonata, Bax's 4 sonatas)
Daniel Blumenthal (Tansman's 5 sonatas and 3 sonatinas, Stanchinsky's piano works, lots of other French pieces)
Marilyn Nonken (an absolute ton of amazing post-1950 music, including works by Finnissy, the complete piano works of Tristan Murail, David Rakowski, and a set of preludes by Dusapin)
Łukaszewski Marcin (numerous Polish pieces including Bacewicz's excellent etudes and pieces by Sawa and Serocki)
And lastly, Ian Pace, of course, who has been involved with some of the most stunning piano repertoire in the world, ranging from Finnissy's monster work "History of Photography in Sound" to Rzewski's "The Road" in addition to loads of other complex and breathtaking pieces of contemporary piano composition, such as Dusapin's stunning etudes and concerto.
I think these folks deserve way more attention and it's brutally tiresome to read post after post of people talking about the same popular players and their zillion competitive versions of the same old repertoire. It's doubly interesting to me when a person is willing to be a music researcher alongside the role of being a music performer.
Same thing with cellists. Sure, Yo-Yo-Ma is an amazing player, but I'd rather listen to a guy like Matt Haimovitz, whose repertoire spreads beyond the standard repertoire to include works by composers like Dutilleux, Hindemith, and Crumb, in addition to more common repertoire like Bach and Beethoven.