I will only mention what hasn't been mentionned already but I support Hogoo's Love definitely.
Marriage Contract - Female lead got pregnant during college, but her husband died, leaving her to cope with his debt and live her life as a single mother and a widow (Heartwrenching 100%).
Angry Mom - High-schooler female lead got pregnant, lives her life as a single mom (this is not the actual plot but that's part of its background).
One More Happy Ending - This story does not deal with a single mother but a single father. Male lead got his girlfriend pregnant during college but she dies during delivery so he raises their child as a single widowed father. The way it is depicted is really unique and beautiful.