Regarding Mozart's church music, I like this Requiem quite a lot, along with his unfinished C-minor Mass KV 427, the Vesperae solennes de confessore KV 339 and the Kyrie in D-minor KV 341.

Above that, Mozart is my favourite opera composer. From Idomeneo on, all his works in this genre, both buffa and seria, are true marvels IMHO.
He also delivered some great attributions to chamber music, like string quartets, quintets, and my personal faves: the piano quartets in G-minor (KV 478) and E-flat Major (KV 493).
To my ears: no lack of substance there.

Anyway: the fact that up to this day serious musicians, musicologists and composers are still busy trying to give Mozart's Requiem a truly Mozartian quality, proves to me that the guy must have had at least a touch of genius.