Definitely Law & Order. I admit I hadn't watched it very frequently in the last few years (mainly due to its timeslot) but I made a point of watching the final episode last night and it was as good as ever.
The other shows, frankly, are flash in the pans. Lost lost my interest ages ago, I never liked 24, Monk never aired properly in my part of the world, and Nip/Tuck wasn't the type of show that interested me, anyway. Law & Order, however, was a generational thing. I was only a couple years out of high school when it started. It has always been there, even if in the background. It and The Simpsons. Yes, it had better years than others, and not every team of cops and lawyers clicked. But the beauty of it is the show kept going, so if you didn't like a certain combination of actors, wait a year or two and you had a reinvention. The only other show that comes close in this regard is Doctor Who.
Not that I expected it to last forever, but I really thought they'd at least get a season 21 out of it, but when the actress who played Van Buren announced she was leaving after 17 years (!) and the LA spinoff was announced, I had a pretty good feeling it was game over.