Having mentioned the term twice now, I ought to define it. (That's the same rule that led me to define reiplophobia, though the usages hadn't been on the blog in that case.) In brief: A definitive production (of, say, Patience) is one that showcases Gilbert and Sullivan, more than the director and cast. It's what some people call a "traditional" production, but that term has so many different meanings that it's largely useless.
Suppose you're involved in a Patience (I'll keep using that example, but the exact same argument can apply to any other G&S - or, for that matter, to any other show), and you're trying to recruit someone into the chorus who's never seen it before. S/he asks you, "What IS Patience?", and to explain, you say: "Get this DVD". Which one do you point to? A definitive one. (In today's world, it'll almost certainly be down to a DVD by the time you're highlighting it in this way, but it'd be even better if you could tell people to "see this live production". Same difference.)
"Definitive" is an ideal that may not be perfectly attained, but the closer a show comes to that, the better it'll be as a showcase production. It's worth noting, by the way, that there can be many definitive works, all distinct and different (and distinctly different), and all worth showing off. This term does NOT mean "slavishly copies hundred-year-old business", or preclude a director's imagination. It just means that the underlying words, music, and characters, are the most obvious.
Grosvenor LOG's Patience was excellent in this regard. Aside from the antics with Jane's cello, which cost us a bit of the music in favour of some hilarity, the most notable aspect of the performance was the way that all Gilbert's jokes were laughed at. That means they were delivered clearly and audibly, with proper timing (comedy is all about timing), and not letting anything get in the way of what was written over a hundred years ago and is still funny today. Yet Grosvenor's Patience was its own item, too; it wasn't an empty vehicle for Gilbert, it had charm specific to itself. The two were not in conflict, which is why I think, though some don't, that it was an excellent show.
I've cited as definitive the professional Ruddigore from two years ago, despite it having a few distinctly off-putting aspects (I really do not see that the ghost scene requires drugs); perhaps some day another Ruddigore will eclipse it (though that one will still be excellent). Lyric Opera's Mikado from this year contends with Savoynet's from 2011. Perhaps one day there'll be a boxed set of superb productions that can be cited as "This is Gilbert & Sullivan", doing what the Brent Walker series failed dismally to do; I would love to see that happen.