The Hosteller & zostel because of location it's in Central delhi. but didn't have the personal experience. these two I can suggest just because of good location. rest you can check online & ratings. 🙏
I would recommend Zostel Delhi from my personal experience but it depends on your location preference. Zostel Delhi is in Pahargunj, and close to the New Delhi railway station. in fact Pahargunj has a lot of cheap and safe options.
Is there any specific requirement? Most of the decent hotels in Delhi are safe to stay. However be a little care while selecting a hotel in Old Delhi/Paharganj area