I am a pretty serious television snob. I mostly watch cable shows like HBO/Showtime shows or Mad Men on AMC and Rescue Me on FX, but I love Supernatural. On network tv, it's pretty far up on the list. I'd say top 5, and seems to get better every season.
I had been thinking that cable shows are better because they don't have such a long season to water down content, but then Supernatural is 22 episode network style and doesn't seem to have a problem keeping the quality up.