I am also fond of season 4 - Stephanie Izard and Richard Blais (and also Antonia, Spike, and Dale), which I think is the first season where everyone was cooking at an insanely high level from really early on. You are correct, however, that Season 6 is the pinnacle of Top Chef on every metric.
I'd add that Blais, Dale, and Antonia truly shone in All-Stars, whose shocking reveal towards the end of Antonia and Mike's relation to each other makes it my second favorite. (Their duel was good clean fun.)
I agree, except that I'd put Kevin WAY higher on the list of pluses. In related news, his cookbook is outstanding.