Wow, apart from Firefly I didn't like any of these. Well, okay, Babylon 5 wasn't bad (at the time) and I didn't watch Veronica Mars.Of the ones currently on the air? Supernatural. It's managed to stay fresh and entertaining for nine years, something very few other shows manage. Justified runs a very close second to this. Legend of Korra.
Of recent shows? Fringe and Medium, both of which got to leave the air with satisfying endings. Young Justice, which did not
Of shows in the past? Firefly, Babylon 5, Veronica Mars (I did not watch the third season), Buffy and Angel.