Babylon 5 - a good, big epic story. 111 episodes should last me just long enough until I'm ready to do a "complete Lost re-watch" when I'll watch all six seasons. B5 would be good in this respect as that's all one big (albeit "incomplete") story, with some truly wonderful moments in it.
Twin Peaks - I've not seen Twin Peaks all the way through in quite some time now. I don't even think I've watched most of Season 2 on DVD at all. It'll be good to revisit it.
The Wire - I started this a little while back, but only got three or four episodes in to season 1 before getting distracted by other things. Having heard so much about it, especially the praise heaped on it by Charlie Brooker, I really should get around to watching it.
There are of course other shows. Battlestar Galactica would probably frustrate me too much, as I'd have to skip some of the really bad episodes and concetrate on the great ones. Oz is a possibility too; Mr Eko's in it. Even The Sopranos; never seen a single episode of that. Some of the box sets of these can be picked up quite cheaply now...
...still, I guess I'll decide closer to the time... : )