South Park and Parks and Recreation are my two favorites. I would add The Office, but for all its many strengths, it ultimately had too many long stretches of just not being enjoyable.
Anyways, in South Park’s case, it’s just the shows ability to constantly deliver excellent, vulgar satire. The show has definitely had its stretches of mediocrity (early seasons, that terrible serialization experiment), but it’s remained pretty consistently good throughout its incredibly long running time. Episodes like Scott Tenorman Must Die, The Return of the Fellowship of the Ring to the Two Towers, and even recent ones like Put It Down are among my favorite episodes in any TV show ever.
I like Parks and Recreation for rather different reasons. Yes, it still has the over-the-top characters getting involved in silly antics, but along with just having some fantastic writing and characters, it still managed to be surprisingly wholesome. It’s a show entirely about government, with highly opinionated characters, and yet it managed to never really stoop to mean-spirited attacks on anyone. Leslie and Ron’s relationship (minus that forced conflict at the start of the final season) sort of exemplifies that friendlier nature of the show. It’s just something I came to marvel as the show went on and made the comedy and characters much more likeable.