Piano only...

The Art Tatum Solo Masterpiece series from Pablo is absolutely essential. He was arguably the greatest soloist in Jazz history, and these recordings will tell you why this is so.

Look for "Bill Charlap" and "Tord Gustavsen". Two tremendous jazz artists.

Also Thelonious Alone in San Francisco and Jessica Williams' All Alone

I have heard neither of these, but have heard enough good things about them (and, of course, have heard their other, non-solo recordings) that I plan to get them myself soon.

Brad Mehldau, Live in Tokyo (or something close to that title)

Also Mehldau's Elegiac Cycle, which I think is even better. As I recall, this has a few tunes with bass and drums, but is definitely mostly solo piano.

Seconding Pragmatist on Art Tatum, yes, Evans also