Ideally you should focus mostly on songs you can get at least an S on, with the occasional A, for the best improvement. And do not avoid any maps that have awkward/frustrating patterns.
These is some benefit to playing maps further outside your comfort zone though. Maps that are difficult to A, or even result in a high B can be good for learning how to not panic/freak out when you encounter patterns you are not used to, can help with stamina a bit, and can introduce patterns you haven't seen before. But it's best to limit how often you play maps like that, because playing them too often can teach you bad habits.
There's a stickied forum thread here that has links to a bunch of packs, including many 7k ones.