His piano sonatas definitely get underrated. I've noticed they don't get advocated for or talked about nearly as much as Beethoven's (or maybe even Haydn's). Yet Beethoven's piano sonatas couldn't have been without Mozart's, and they are actually quite similar in many ways. So, how can anyone say they love Beethoven's Piano Sonatas, and not Mozart's? That doesn't make any sense. So too are Mozart's string quartets, string quintets, piano trios, violin sonatas, violin concerti, Sinfonia Concertante, choral music, and operas all underrated, in my view. Maybe his piano concertos, Requiem, and symphonies get fully appreciated--just an impression.
If pressed, I'd personally rank Don Giovanni as the greatest opera ever written, and The Magic Flute not far behind it.
But over the years, I've noticed that mere mortals often under-appreciate super genius. I guess it all sounds more effortless and easy than it actually is.
Someone made an analogy to tennis player Roger Federer earlier in this thread, and I agree. Federer makes what is incredibly difficult look so easy and natural and effortless. Non-top tennis players have no idea how phenomenal Federer is on a tennis court. His resting heart beat is in the 30s, his hand eye coordination is beyond belief, and he's super quick and fast, with a massive forehand (& everything else, too, including a Pete Sampras-like vertical leap in his younger days--47-48"-which was on par with Michael Jordan's, btw). While throughout most of his career, not a hair would be out of place on Federer's head deep into a 5th set (that is, after four or five hours). He wouldn't even look like he was sweating or remotely tired. That's simply unbelievable (& freakish), to anyone that has actually played a grueling 5 set tennis match. Indeed, Federer makes it all look so easy that the average tennis viewer doesn't fully realize or appreciate that what Federer is doing is extremely difficult and rare, and not easy at all, in fact, just the opposite.
& I think the same is true for Mozart, too. I expect top flight musicians know this. You can't play this music very well and not realize the super genius behind it; that is, unless you're Glenn Gould...
But that's just it, I'd say the piano concertos and symphonies get played consistently well, for the most part, but the same isn't as true for many of Mozart's other works. For example, I'm still waiting for a recording of Don Giovanni that truly does the score full justice.