Who did Ron Weasley have a crush on in Harry Potter?

Ron Weasley had a crush on Lavender Brown for a while in the sixth book, "Harry Potter and the Half-Blood Prince". He even started dating her, which made Harry and Hermione very uncomfortable.

However, Ron's feelings for Lavender were ultimately shallow and based on her popularity, and he broke up with her later on. His true feelings were always for Hermione Granger, although he took a long time to admit them.