Sword art Online
The only one i could think of at the moment. While it is not the absolute worst anime I've seen its gotta be in the top 5. The show starts out really really well and it sets itself up for alot of potential and a great show. That is until Kirito Meets Asuna... Then the show is just focused on love and the whole "Damsel in Distress" cliche. At the point everything the show was based off of in the beginning is pretty much thrown out of the window in my opinion.