Book Series Recommendations?

I mentioned earlier this week that I was reading 50 Shades of Grey. Well, I have finished it and I'm on to book two! I had dinner with a friend last night and I was helping her think of other book series that she should read.We both agreed that we prefer series because if we finish one book and we want more...there's more! So, my question for all of you is...Besides the obvious Twilight, Hunger Games and 50 Shades of Grey series, what other series of books are you loving??