There’s no point in doing a complete survey, but here are a few observations and suggestions:
1. I am not intrigued by much Mozart written before K330 or so. Piano Concerto #9 is one exception to this. But Toscanini was right to claim that too much of it sounds the same.
2. The string quintets are the best Mozart pieces you might not know, but skip K174.
3. The string quartets and Requiem might be the most overrated Mozart, though the latter would be wonderful if he could have finished it. It is better to listen to the fragmented version, without the artificial Süssmayr ending.
4. The Milos Forman Mozart movie is worth a viewing, if you don’t already know it. I thought I would hate it, but didn’t. Don’t try to learn history from it, however.
6. The operas reign supreme. Try Currentzis or Colin Davis for Don Giovanni, Haitink or Klemperer for The Magic Flute, Boehm for Cosi Fan Tutte, Giulini for Figaro, and Rene Jacobs for Idomeneo. I don’t know of a definitive version of Abduction from the Seraglio, but Beecham and Krips are good and Harnoncourt does the overture best, as he never lets up on the rambunctious in it. If I had to choose the operas, or all the rest of Mozart put together, I would go for the operas.