Mt. Diablo SP has 3 campgrounds: Live Oak, Juniper, and several first come first serve sites at the junction of South Gate and North Gate roads about 2000ft up the mountain.

Live Oak is the closest to the entrance (and town), and features some cool rock formations to explore, but it's colder, has terrible cellular data service, and doesn't quite have the sunset / sunrise views that Juniper has.

Juniper is way up there, plan for a 45 minute drive from town to the top, longer if you are doing it on a weekend when you'll be waiting to pass cyclists (be very careful, do not pass on blind corners, this is a cycling destination). But it is absolutely worth it for the views of the east bay, and on a clear day, all the way to SF and the pacific. Truly one of the best camping views I've had.

Juniper had decent 10-15mbps ATT cellular service in most site locations, and also decent t-mobile, but ping was slow. The best sites at Juniper if you're sleeping in your vehicle are the flat ones down by the parking lot and vista (18, 20, 22). Note that most of the sites on the west side are not flat for a vehicle. Fine for a ground tent. Lot's of wind up this high, and not a lot of shelter, so plan accordingly. Water is off, probably forever as CA will be in perpetual drought, so plan for that as well.

Only reason for 4 stars is that they lock you in at sunset; if you want to leave you have to call the fire department to open the locked gate, it's not like a tiger-teeth exit but no entry kind of situation. This significantly reduces flexibility, but I can understand why they don't want a bunch of cars trying to race up and down the mountain after dark.