When CBS moved Match Game to the mornings in 1978, that all but killed it, ratings-wise.
That would be on my list too. Some of the moves of the mid-late '70s didn't thrill me, as it meant either missing a show because of school, or having to choose between two shows. When
Tattletales moved briefly to the mornings in '75 it was just before summer vacation. It meant I had to choose between that and
High Rollers (back in the days before VCRs).
Celebrity Sweepstakes moving from 12:30 PM to 10 AM on 1/6/75 wasn't good either - I was a big fan of that show and watched it every day at lunch. When it moved, I was out of luck. Eventually, the NBC station in my area expanded
Dinah! (which they ran at 9) from 60 minutes to 90 minutes and even when I was home from school I couldn't see it. Luckily when it moved to 10:30 I could see it again.
Sweepstakes ranks close, but probably No. 1 for me is CBS moving
Press Your Luck from 10:30 to 4 PM in January 1986. Very few stations cleared the 4 PM slot in those days and it probably meant ratings for the show took a nosedive. Luckily many of those '86 episodes popped up on USA in the early '90s.
PYL's move was made in favor of the revival of
Card Sharks. All through the '70s and '80s Mark Goodson seemed to carry a lot of clout at the networks - from what I've read it seems that almost everything he wanted, he got. Apparantly he didn't want
Card Sharks going in at 4 PM because
Body Language wasn't very successful there. The ratings of PYL were dropping by fall '85 anyway, but the move to late afternoons quickly killed the show. It barely hung in until Sept '86 when CBS gave the slot back to affiliates.