Cryogonal, by a long shot. I saw its picture and thought it was going to be this gen's Ice-type beast (like Glalie in Gen III) and then there was the fact that the chances of meeting it in the wild only go as high as 5%. I got super excited over it and had great plans for it. Then after nearly 5 hours in Twist Mountain I got one and it was such a let down. Its Sp. Atk was ok but it was useless in every other stat.
The three monkey pokemon, obviously. They quickly outlived their usefulness.
Druddigon. I didn't like its low speed. I make it a rule that if a Pokemon needs a Quick Claw to go first it has no place on my team. I only broke that for the Tirtouga line who, unless you choose Oshawott or Snivy, are the first water types you can get (I think).
I do have to disagree with a lot of the others though. I found Zebstrika to be a great pokemon, even though I replaced it with Joltik/Galvantula it served its purpose well. Purrloin is this Gen's Poochyena, don't expect it to work wonders and cr*p miracles