Being an exclusively Precinct 1 cop, the best I've ever seen has actually been Captain Jamooze. Anyone can RP well if they put effort in but when I've seen this guy really try to RP it doesn't feel forced. Maybe its just his soothing voice or something but jesus, it feels more real than other people.
To be fair, I've really only processed him like twice, so maybe I just caught him during his best.