Buffy the Vampire Slayer, countless rewatches. One of the best shows of the 90's/00's. Character development and fulfilling storyline threads throughout.
Gilmore Girls is easy comfort TV, some comedy, some drama, generally low stakes and just cozy to watch, so I do, again and again.
Friday Night Lights is full of endearing characters and good natured drama. I hate football but I love this show. I think I've watched it through five times now.
Firefly didn't last long enough to make a bad episode, all of the series is excellent and rewatchable. So I rewatch it, lots.
Veronica Mars I've watched through five or six times, it has great dialogue and great relationships to follow through interesting arcs.
30 Rock is one of the few half hour comedies I find easy to rewatch, several times now. Most comedies feel generally too mean spirited for me anymore.
Cowboy Bebop is worth a go every few years, the rule of cool.
Deadwood is just so good, even though it's kind of heavy, it has such excellent acting and storytelling that I'll go back to it every few years. Usually I can't do that with high stakes dramas, but this one got everything right (except being cut short and having to be finished many years later)