Ichiban Ushiro no Daimaou was one of my first anime to research, watch online, and look forward to every week. I would say it was the bookmark that changed me from "I'm watching anime" to "I'm an Anime Newb". It was a phenomenal anime.
It's full of hilarious moments, perversion, and tons of action. The anime follows Akuto SAI in a world rich with mana where magic is taught in schools. Unfortunately, out of the gate Sai is given a bad rap. As he attempts to clear his name throughout the entire anime, people are constantly confronting him and making assumptions. It's often quite funny to watch as Sai tries to dig himself out of deeper and deeper holes in each situation.
The music and sound were quite enjoyable and exactly on sync. The opening is one of my favorites, which you can find here.
The characters are all love or you love to hate them. This anime will end each episode, with you digging for the next episode out of anticipation.
I highly suggest everyone take this series on. I leave you with this:
That's worth 10/10 alone.