I know not many people do it, but I have plenty of time on my hands (well, not lately due to exams). I usually rewatch anime after reading the manga. This allows me to compare certain things.
Plus, you get to see things from a different perspective and become more observant towards stuff you may have missed or did not understand. You'll be surprised at what you find.
Anyways, if you had to rewatch anime, what would it be?
I rewatched this month:
Mai-HiME
Negima! (because season 2 subbing is going slow and I need my fill)
Ouran High School Host Club (haha, so ownage)
I also plan to rewatch Code Geass (at least the last 10 episodes) before 25 and 26 comes out, else I won't get in the mood >_>"