Absolutely love Michael Mann. A massively understated talent who deserves far more recognition. For me hes second to only Scorcese when it comes to the modern crime genre.
But sorry, back to the question. For me its Heat, its what he does best: crime, two characters coliding, and presenting L.A in a dazzling manor.
its probably followed by the insider. Also love collateral and public enemies and Im still yet to watch thief and manhunter, which I have recently bought and they are meant to be really great