I like to watch all of them whenever they're on TV.
But to be honest, I hate what the Simpsons has become, I've been rewatching Season 5 and it's golden, but today, Mike Scully continues to wreck the show.
Family Guy is by the books a bad show, but it's kinda entertaining, like you're sitting next to your best friend and you're too happy to criticise the fact you're both doing really stupid shit. It's a guilty pleasure.
South Park is pretty damn fine, but the newer episodes became too formulaic and predictable. All of their jokes follow the same stages, switching between two or more parallel narratives, usually 3-4 times, and progressing the single joke of the episode. The older episodes were much more creative like a child's unbridled creativity, excited to be making a TV show.
Futurama is hard for me to explain, it's just great. The movies and the latest episodes are also entertaining but a shell of their former selves. It might unfortunately go the way of the Simpsons, like how it started falling off in season 10. But for the time being, I can enjoy the new episodes.
There's also King of the Hill, which I applaud for trying to be something more, it's like reading a novel on modern times.