Honestly... none.
I never owned any English study guides and I don't regret it either. In English, you really need to stand out and form your own unique and meaningful interpretation. That won't be achieved by looking at study guides that everyone else in the state has access to.
What I did was read my texts thoroughly -- I don't mean several times though -- and I tried to question the motives of the characters, what was happening/why, etc etc. As soon as I had a thought I would write it in the margins of my text (if you have a film just write notes). By the end of the year I had my own study guide that no one else had, full of ideas that I could use to tackle any prompt.
Also, I loved discussing my texts with teachers, AN'ers, friends, anyone who wanted to listen because their interpretation is definitely valid and can really give you some quality material to work with. If I picked up anything from someone else I'd add it to my crazy annotations and give it my own spin for later use.