Ever, I'm going to have to go with The Twilight Zone. The fact that they're still interesting 50 years later says a hell of a lot.
Currently...? Hm. A year or so ago, I'd have said Lost. Now..? Tough call. Probably my favourite new show is Law & Order UK. I'm really in love with it, and dying to see the next episode tomorrow (OMG). It also captivates me because it's old original Law & Order eps from 10+ years ago (mainly, the awesome Lenny years), updated to the 21st century and transferred from NYC to London. Even the most classic eps, like the one where Rey takes the woman whose baby has gone missing back through her day to retrace her steps and it ends up she put the child in a furnace so it could be in heaven...they're just different and fresh.
And, okay, yeah, I love playing the "how long does it take to identify the episode" game. A lot.