The Operas: The peaks of his achievement. For Figaro I recommend Carlos Guilini or Rene Jacob, for Cosi Fan Tutte, Karl Boehm, here is my post on Don Giovanni, and Klemperer is a sure thing for The Magic Flute. For The Abduction from the Sergalio, how about Beecham with a nod to Krips?
The String Quintets: Grumiaux’s group, with Takacs as a good runner-up. Most of the string quartets are boring.
Symphonies: I am courting hate mail, but 38-41 will suffice, toss in the first movement of 29 if need be. I like von Karajan for the last two symphonies (not everyone does), and there are many good versions of the others.
Piano Concerti: Focus on 20-27; I grew up with Casadesus and Szell but you have many good choices. Few areas of the repertoire have been better covered.
If you own these you have a decent chunk of the essential Mozart.
Most overrated Mozart: The Violin concerti and then the Requiem. Contrary to cliche, Suessmayr ruined the ending. The Clarinet Concerto was once wonderful, but it has been overexposed in muzak, Nordstrom, and overpriced faux Italian restaurants. By the way, it won a listeners’ poll as "best Mozart," the Requiem came in second.
Most underrated Mozart: The violin and piano sonatas, and the short, comic vocal pieces. Try also the Piano and Wind Quintet, K. 452, the Clarinet Trio, K. 498, the Piano Quartets (with George Szell as pianist), and the Clarinet Quintet, K. 581.
Comments are open, do offer your opinions…