to just mindlessly re-watch something, I'd say most sitcoms fit the bill. contained storylines, memorable characters, plus the familiarity to go "Oh, is this the one where...?" and then sit down and watch it. I do this most with Seinfeld and Friends, but also Arrested Development (which I have to specifically seek out to turn on, rather than the syndicated ones which are just always on), Full House (yep), Entourage, Fresh Prince, King of Queens, Modern Family, The Office, etc. I'd also include action/crime dramas - Criminal Minds, Burn Notice.
If I were to deem a whole series rewatchable (as in, I would start at S1, E1 and watch it all the way through), I'd say West Wing and LOST. I haven't seen The Wire but intend to watch it soon and feel like it will fit that bill.