CSI (LV) ruined when Danson and Shue took over. Not so much by the actors as by the incredibly lazy writing.
L&O: flashbacks to watching it with my ex wife. Seriously bad associations.
CSI:NY when they stopped being about solving crimes and made it more of a soap opera. Really the worst was when the main character admitted to knowing that a cop had killed a helpless suspect, and decided to do nothing about it. And it was an out-of-body experience episode, to really terrible writing.
Bones season 8. The pregnancy in season 7 was bad enough, but the overall writing in season 8 is just an abomination that reaches back to the earliest episodes and tries to strangle the show in its crib.
Series 6 of Doctor Who: again, the holes in the writing, and elements that just don't make any sense.
Torchwood: MD
The series had its flaws, and CoE was pretty terrible, but MD was the tombstone, followed by the backhoe exhumation, followed by an autopsy, cremation, and re-interment.
"Give me Manual Control" from one of the Next Gen movies.
"Mother, I insist you get naked, this instant." from a season3 DS9 episode (one of two grotesque abominations of stories from that season).
The opening credits from Phantom Menace for the entire prequel trilogy.
Season 3 of Clone Wars for the entire Clone Wars cartoon series.
Highlander: The Source.
Tracy Scoggins cast in anything.
Michael Bay as director of anything.
Bumblebee peeing on a federal agent.