Mt. Lassen, Summit Lake South is clean, quiet and beautiful! Because it has fewer improvements (pit toilets, no electricity, and smaller vehicles - and some tent-only areas), it makes for a fantastic outdoor experience. The campsites are well-positioned to provide a certain level of privacy, but close to lake and trails leading to Kings Creek, Warner Valley and Bumpas Hell. All accessible without having to drive to the trailhead. We especially love going swimming (or floating) on Summit Lake after a good hike, in the late afternoon. It's cool, but after a few minutes you feel fine and the views are amazing as you look up at the trees and peaks. It's also significantly less noisy or crowded (compared to the North Campground and picnic area).
Nights are also fantastic for stargazing, and my kids and I like to lie down on the campsite road (around 11pm, when there are no cars) where you still feel some warmth, and look up at the stars and Milky Way. Stunning!
As a comparison - we have camped at Manzanita, which has more amenities (a camp store, firewood, fuel, etc.), but it's a lot more congested with people, cars, campers, trailers, etc. So, it all depends on what you want! Enjoy.