Shakugan no Shana.
I mean like fucking hell, I had too much time on my hands. It was about 2 years ago since I watched it so my memory is a bit hazy, but I can definitely remember how boring and shit it was. Season 1 was confusing but different from usual animu, so I kept watching. Generic if anything, but still ok I guess. Season 2 was completely boring, but I somehow finished that. I was hesitant to watch season 3 but I didn't really want to leave an anime unfinished halfway through. Then season 3 was a complete clusterfuck, consisting of a boring plot and introductions to 20 different irrelevant characters each episode that you were forced to somehow remember. Each to their own, but fuck me this was the worst anime I've ever watched.