All just my personal opinion
Beethoven: Piano Sonata No.26 in E♭ major, op.81a, “Les Adieux” — comparing recordings with the following artists: Wilhelm Backhaus (1961), Paul Badura-Skoda (1969), Daniel Barenboim (1984), Ronald Brautigam (2007), Emil Gilels (1984), Friedrich Gulda (1967), Arthur Rubinstein (1940 and 1962), Artur Schnabel (1935)
Comparing recordings with the following artists: Wilhelm Backhaus (1966), Paul Badura-Skoda (1969), Daniel Barenboim (1984), Ronald Brautigam (2008), Emil Gilels (1985), Friedrich Gulda (1967), Igor Levit (2013), Maurizio Pollini (1976), Svjatoslav Richter (1965), Artur Schnabel (1935)
Beethoven: Piano Sonata No.32 in C minor, op.111 — Comparing recordings with the following artists: Wilhelm Backhaus (1961), Paul Badura-Skoda (1969), Daniel Barenboim (1984), Ronald Brautigam (2008), Friedrich Gulda (1967), Julius Katchen (1968), Wilhelm Kempff (1964), Igor Levit (2013), Maurizio Pollini (1976), András Schiff (2012), Artur Schnabel (1935)
Beethoven: Piano Sonatas — summary for comparisons of recordings, involving the following artists: Wilhelm Backhaus, Paul Badura-Skoda, Daniel Barenboim, Ronald Brautigam, Edwin Fischer, Emil Gilels, Friedrich Gulda, Clara Haskil, Jos van Immerseel, Julius Katchen, Wilhelm Kempff, Igor Levit, Mikhail Pletnev, Maurizio Pollini, Svjatoslav Richter, Arthur Rubinstein, András Schiff, Artur Schnabel, Mélodie Zhao
Beethoven: Piano Sonata No.22 in F major, op.54 — comparing recordings with the following artists: Wilhelm Backhaus (1969), Paul Badura-Skoda (1969), Daniel Barenboim (1984), Ronald Brautigam (2007), Friedrich Gulda (1967), Maurizio Pollini (2003), Artur Schnabel (1935)
Beethoven: Piano Sonata No.15 in D major, op.28, “Pastoral” — comparing recordings with the following artists: Wilhelm Backhaus (1961), Paul Badura-Skoda (1969), Daniel Barenboim (1984), Ronald Brautigam (2005), Emil Gilels (1982), Friedrich Gulda (1967), Artur Schnabel (1935)