Some of the most grossly overrated pianists in my opinion (save from taking offence my friends, if you happen to be their fans - this is just my opinion):

Deceased:
1. Vladimir Horowitz. Believe me I have never liked any single piece he has recorded or played live that I have ever heard, except Russians like Skriabin. Flashy, showy technician too eager to prove himself - '
See? I can do anything better than all you guys!'
2. Artur Schnabel. Badly played Beethoven. Almost hurts my ears - just wondering if his fingers were diminutive hammers. How he earned his fame of the 'first Beethoven authority' remains a mystery to me.
3. Glenn Gould. Er . . . Bach for Beethoven, Bach for Chopin, Bach for everybody else?
4. Sergei Rachmaninoff. Oh well, he didn't even convince me with his own set of Concertos, let alone Chopin. Listen to his Ampico-roll reproduction of the Chopin
Scherzo in Bb minor, Op.31 and you'll get a flavour of what I call infuriating performance.
Living:
1. Martha Argerich, hand-in-hand with Ivo Pogorelich (and yes, I have seen Pogorelich live, shook hands with him, and got his autograph). Clearly these two pianists put too much of
themselves in the music they play, not uncommonly to the point of distortion. Technically yes, they are among the world's Number One, but so what, if they don't sound musical?
2. Marc-Andre Hamelin. Again one of the supremely impeccable pianists, but then the music?
Now on some quite underrated living pianists:
1. Alicia de Larrocha
2. Cecile Ousset
These are outstanding ladies who easily outperform musically an Argerich and even many great male pianists. Ousset's recital in Hong Kong back in the early-mid 1990s captivated me in awe with her programme, particularly so with Ravel and Liszt. Technique and music alike. Great thing about them is that they don't put too much of themselves in the music they play.
3. Leslie Howard. His Liszt is not only absolutely the most comprehensive, but also very good and convincing overall. Now he could move on to other composers.