Mozart, indeed; among his recordings is a superb Idomeneo, with Barbara Frittoli, Ian Bostridge, LHL, and Anthony Rolfe Johnson. But the composer with whom I most closely associate him isn't Sir Arthur Sullivan, it's Leoš Janáček. Mackerras was as responsible as anyone for Janáček's entrance in the standard repertory, between his editions, recordings, performances, and advocacy.
I was lucky enough to hear Mackerras a few times at San Francisco Opera, in Vec Makropoulos (my first Janáček, and yes, it made quite an impression), in Rusalka, the Rosenkavalier I walked out of in sheer annoyance (we arrived on time for the imagined 2 p.m. curtain and had to stand through act I) and in the splendid Semele of a decade ago. I heard two performances of the Handel, one with Mackerras and the other with William Lacey. At this distance in time and without notes, I can't give you the specifics, but the two demonstrated nicely the difference between the great and the good.
Rest in peace, Sir Charles. You were one of the greats.