I realized the other day that I never used to watch series at all, except of course the kid-classics like Looney Toons, Tom & Jerry and Thundercats. Since 2007, though, I've acquired quite a big list of series I adore so much that I've watched every episode, most of them several times.
Such as (in no special order): Frasier, Gilmore Girls, How I met your Mother, Big Bang Theory, Wildfire, Black Beauty (I feel the guys smirking. Whatever. I like horses. Sue me.), Scrubs, Heroes, Alias, Only Fools and Horses, Open all Hours, Allo Allo, Modern Family, 30 Rock, Friends, Buffy, True Blood... I think that's it. The last five series I'm not quite as familiar with as the others, but I think they still count.
What impresses me about series like these is the (mostly) consistently great screenwriting. The eps are funny and entertaining, something new every week, something I hunger to experience and recreate - the former, sadly, being enormously easier than the latter. So much work must go into the scripts, I can't even imagine...