While there are several pianists that I would submit are overrated, I definitely would not place Horowitz in that category. Here's a highly subjective list of those pianists which I consider UNDERrated:
Nelson Friere--known usually as Argerich's 2nd piano buddy, I'd quite frankly pay to hear him play, not her.
Jorge Bolet--unfortunately, he got famous when his playing declined and it's difficult to find his great recordings (live especially) from the 60's-70's.
Cziffra--He could do much more than just play Liszt really fast.
Emil Gilels--Everyone seems to talk about Richter and somehow neglect Gilels, strange.
Anton Kuerti--alas, his recordings only give an indication of the fire he can conjure up during a live performance.
Byron Janis--always seemingly in Horowitz's shadow, he was a formidible pianist in his own right, the 60's Mercury recording attest to that.
Ernst Levy--not a household name, but an artist of astounding depth and originality.
Vladimir Sofronitsky--was never allowed to cross the Iron Curtain. Son-in-law of Scriabin; when he was on, no-one played Scriabin better.