I watched Haibane Renmei a few days ago. I can't really tell yet if this is going to be the case, since my gut feel has a natural bias towards things I've seen more recently, but it may well become of my favorite animes.
I think there are a lot of fresh concepts and I find the slice-of-life feel of the series very immersive. There are many unanswered questions left for interpretation: the characters' origins, the geography, the rules, all contribute to provide a rich culture to the series' world. Exploring the mysteries unravels interesting theories that can add depth to the world on their own.
It's all very fascinating and stimulating. It's not very often that a series makes me pick up creative writing again. :)