Over-rated: John Ogdon, Evgeny Kissin, Michael Ponti, Van Cliburn, Georges Cziffra
As for under-rated, it depends on the performance. Robert Taub is definitely under-rated when it comes to some Scriabin. His complete sonatas collection is the best I've heard, but not every performance is outstanding. The 8th and 9th aren't terribly compelling, for instance. I've heard his Beethoven isn't the best.
Bernd Glemser is probably under-rated, because some of his Scriabin sonatas are the best I've heard, like the 7th. But, his first disc of them is better than the 2nd.
Anyone who says Horowitz is overrated must refer to specific performances, because--overall--he is a genius. Some of his performances are not the best, because he's human.
Richter, like Horowitz, produced some unimpressive performances as well as outstanding performances.
As for Rubinstein... I've never heard a better version of Chopin's G minor ballade and don't expect to. Horowitz's definitely doesn't do the job. Rubinstein also did the best I've heard with Brahms' second concerto (a 1960 live recording in Warsaw). His Beethoven sonatas are solid and so is his Chopin.
Ashkenazy is uneven. His performance of the Rachmaninov unrevised second sonata is the best I've heard, and he did a wonderful job with Prokofiev's 2nd concerto and Scriabin's 1st concerto. Yet, his Scriabin sonatas are not the best (although he has the best 8th I've heard) and I have a disc of him playing Beethoven sonatas that is vastly inferior to Rubinstein and Richter. To Ashkenazy's credit, a lot of pianists have butchered that Rachmaninov sonata. The only issue I have with his performance is the unusual chord near the end of the second movement, which is either due to his mistake or a mistake in the score.