It's my favorite of the series, though I never played V.
San Andreas struck a great balance between serious and ridiculous. Some of the gang rivalry story hit pretty hard for its time, but also there was a mini Las Vegas and a goofy jetpack and tons of dumb Cluckin' Bell jokes. It even had RPG elements like CJ getting more fit if you work out or fat if you fed him tons of fast food. Not everybody loved this and not surprisingly it was cut from later entries but I thought it added to the world. And the map was great, lots of variation between city and countryside. Great soundtrack too, rivaled only by Vice City IMO.
IV just didn't do it for me. The phone (cousin Roman!) was obnoxious, the cars felt too heavy, the story just wasn't very funny, and the map was pretty much all city.
Maybe I'd enjoy V since it's huge and supposedly the vehicles are more fun, but I hear the story isn't great.