As I am currently re-watching the first season, I'm going to have to with
Twin Peaks. While parts of the second season are admittedly slow, and well, kind of boring and pointless, the first season and end of the series is probably the most brilliant thing ever put on television. David Lynch is a genius, and the show was only great when he was truly interested (hence the dip the show took mid-second season when Lynch lost interest). It's a shame the show was canceled, as it had a lot of potential...Looking back at it now, it's hard to believe that this show was on network TV back in 1990 as it was way of ahead of its time and probably its audience as well. Anyway, the show is only two seasons, so I highly recommend you check it out...Definitely worth it.
Runner-ups would probably be The Wire (could probably re-watch all 5 seasons again soon) and Arrested Development.
Edit: Forgot to add that the many interpretations that can be made from Twin Peaks would make it a great box set to have on a desert island. There are many theories and ideas about what certain things mean in the show and I'm sure if I had time on a desert island a 100+ page thesis could be written about it...
Also, I haven't seen the show, but wouldn't having LOST on a desert island be just a tad tacky?