Some say every arc except the Zabuza Arc, others say every arc after the chuunin exam. I myself am a bit more generous, though, and would probably say everything after Gaara vs Kimimaro excluding Shikamaru vs Hidan.
Don't be mistaken, I'm not saying there's nothing good in those arcs. In order to "ruin" an arc, it has to start out with promise. Most of the shippudden arcs manage that, but something dissappointing always seems to happen near the end to ruin it.