When news broke a week and a half ago that NBC had elected to cancel "Law & Order" - and to do it one season shy of passing "Gunsmoke" to claim the title of longest-running primetime drama series - I began thinking back over the more than two dozen actors and actresses who have been regular co-stars on the series over its 20-year run, and I tried conducting a mental fantasy draft of "Law & Order" characters. If you kept everyone in a job they had held at some point during the series (i.e., Jack McCoy could be the lead prosecutor or the Manhattan DA, but no fair making Lennie Briscoe the precinct lieutenant supervising partners Max Greevey and Phil Ceretta), which combination would be most interesting?

As I pondered different groupings (and read your own), I came again and again to the same conclusion: the season five cast (Briscoe, Mike Logan and Anita Van Buren as the cops, McCoy, Claire Kincaid and wise old Adam Schiff as the lawyers) was so perfect that I didn't even need to do the fantasy thing. (Though I did think it would be interesting to see McCoy as the DA and Ben Stone as the prosecutor, both because the two would butt heads and because Stone would not be pleased to realize Jack and Claire were having an affair.) Not only were most of these characters the best individual examples of their respective position on the show (surely, any "L&O" fantasy draft would begin with Briscoe as senior detective), but they all worked so well together.

Then the mental game took a different turn. If there was no point to constructing a hypothetical lineup of cops and prosecutors, then what actual cast would rank second overall to that season five Murderer's Row?

My answer: the one you'll get to see for the last time tonight.

(There are rumors that TNT, which has made a small fortune from airing the show's repeats, might step in to order one last season just to give "L&O" creator Dick Wolf the bragging rights over "Gunsmoke," but even if that happens, I would imagine there would be a reduced budget and the current six wouldn't return en masse.)

Are these six - Cyrus Lupo (Jeremy Sisto) and Kevin Bernard (Anthony Anderson) as the detectives, Van Buren (S. Epatha Merkerson) as their boss, Michael Cutter (Linus Roache) and Connie Rubirosa (Alana de la Garza) as the prosecutors working under McCoy (Sam Waterston) - all the best to play their respective positions? Not all, no. But there also aren't obvious weak links the way there have been in almost every season other than the fifth.

The first three seasons had Don Cragen as the lieutenant and Paul Robinette as the assistant DA, and nobody much liked either character until they returned years after they originally left. (Cragen has been much more popular as the boss on "SVU.") Logan was replaced by the bland Rey Curtis. The post-Robinette assistant DA spot has proved that all supermodel-looking lawyers are not created equal, with strong characters like Kincaid and Abbie Carmichael, but also complete duds like Serena Southerlyn and Alexandra Borgia, both remembered almost entirely for their exits. (Borgia was murdered in gruesome fashion, while Southerlyn asked the infamous question, "Is it because I'm a lesbian?" when she was fired.) Actors Dennis Farina and Dianne Wiest both seemed like obvious choices to join the cast and succeed beloved veterans, but both had uninspired two-season stints. Etc.

This current group, which came together midway through the 18th season when Bernard succeeded Jesse L. Martin's Ed Green, doesn't have anyone who inspires questions of "What is this person doing on my show?" The actors all play well off of each other, and the character dynamics have added a nice wrinkle to a very old and familiar formula. In particular, the idea of promoting McCoy and filling his old job with Cutter - who, though neither man will admit it, is a younger, smoother version of the Jack we watched for 13 seasons - breathed new life into the legal half of the show so the series didn't have to rely as heavily on all those "Don't miss the last two minutes!" plot twists.

It's a very good group, put on strong display in tonight's episode, which wasn't written as a series finale (though it does shine a nice spotlight on Van Buren, since Merkerson wasn't planning to return even if the show had been renewed). Like all the episodes I've seen with this current cast, it made me lament the fact that I didn't watch the show nearly as often as I did in the series' glory years, or even during the spotty middle seasons.

But I didn't watch more often for the same reason that most of you didn't, and which is why the show's ratings are now so low that NBC couldn't renew it even as a gesture to Wolf: when a show has been around as long as this one has, and when the show's many, many episodes - not to mention those of its spin-offs - are available seemingly around the clock on one platform or another, the sense of urgency to see the new stuff isn't there anymore. And the fact that the show did struggle so much with casting in the middle years, particularly after Orbach left, certainly didn't help in convincing any expatriates to give the new gang a try.

Much has been written over the last 20 years on the key to the series' enduring success, particularly as Wolf and company have kept a revolving door going for the cast. Some posited that the true star of the show has been the writing, while others say that New York is the only character that matters. (NBC, which is placing the "Law & Order: LA" spin-off in the mothership's old Wednesday timeslot next fall, had better hope New York isn't that crucial.)

But I think the actual "Law & Order" characters matter at as much or more than the setting or the plots. That the show was able to thrive with so many cast changes doesn't mean the franchise's fans didn't care about the individual people. After all, "Law & Order: Special Victims Unit" is finishing its 11th popular season with the same two leads, plus two supporting actors who have been around from day one. It means that the producers, particularly in the early days, were really good at casting and writing for replacements when original people had to go for one reason or another. When Paul Sorvino quit, allegedly to protect his singing voice, the producers got Orbach. When NBC ordered Wolf to bring in some female actors, they fired Dann Florek as Cragen and Richard Brooks as Robinette and got Merkerson and Jill Hennessy. Benjamin Bratt was never a great fit as Curtis, but successor Martin had fine chemistry with Orbach, etc.

At the start of every episode, the narrator talks about the cops and district attorneys who represent the citizens of New York, always closing with the line "These are their stories" - and "their" is as important as "stories." If "Law & Order" has to end its record-tying run, it's nice to know it could do it with two separate but equally compelling groups.

Alan Sepinwall may be reached at sepinwall@hitfix.com