Well, Mozart's Requiem is hardly a piano piece, but most of his piano sonatas have slow movements which I'm sure would suit. Mostly these are the second movements, for example of K279, 280, 281, 309, 310, 330, 332, 333, 457, 533, 545, and 576, in addition to K570 which Monica mentioned. Also the 1st movements of K282 and K331 (though the latter is only slow in places - these are the lovely variations in the sonata otherwise famous for the Turkish march). My favourites from the above list are K280, 330, 331, 332 (but this has one bar containing a rather fast run, if you play the 1st edition version rather than the manuscript version), and of course the charmingly simple K545. The thing to remember about Mozart is that although much of his stuff is fairly easy to play "correctly" (in a mechanical technical sense), it's not all that easy to play well (in a musical sense). There is much emotion lurking in there, dying to be let out. You just have to feel it deep inside you, it's something you either have or not, I don't know if it can really be "learned". It's so easy to play Mozart badly despite all the notes being in the right place.