I like the non-Wexford psychological thrillers written under her own name. Very similar to Patricia Highsmith in how you get completely pulled into the mindsets of obsessive characters as the tension ratchets up. The Bridesmaid and Live Flesh are the best of these, but she wrote quite a few. Barbara Vine novels are good too - also psychological thrillers but have more of a focus on the relationships between people and buried secrets, whereas the RR thrillers are more about what happens when someone who has lost touch with reality finally goes too far. My favourite Vine is A Fatal Inversion, and I loved Grasshopper too. Both great, intricate plots.
If you like short stories with a twist, RR wrote some fantastic ones. There are a couple of collections.
I wasn’t keen on her later work - I think her peak was in the 90s.