I'm going to say the Bount arc. It was interesting and the story was pretty cool but, I couldn't help but, feel a little bored with it. At some points I felt like I had to force myself to watch it because it felt as though there were parts where it got REALLY boring for me but, being the kind of person who refuses to skip ANY part of any Anime no matter what, I pushed through it~
The Lost Substitute Shinigami Arc... That was so boring... I had to wait a week for each episode to come out and it just wasn't worth the wait. The Gotei 13 Invading Army was a bit average, too. However, I enjoyed all of the Bleach anime, up until the Arrancar: Downfall Arc finished.