My least-favorite Peanuts specials are What a Nightmare, Charlie Brown, SOmeday You'll Find Her, Charlie Brown, It's Flashbeagle Charlie Brown, What Have We Learned, Charlie Brown?, and Life is a Circus, Charlie Brown. I'd like to say that I also dislike most of the specials from the '90s and later, but I don't think I've watched those enough times (the 2000s specials in particular I've seen an average of one time each). I'm also not too big on the This is America, Charlie Brown specials, with the exception of the Smithsonian episode, the Nasa episode, and the music episode (I don't even care to check on the titles).
My least-favorite Garfield special is Garfield Gets a Life. Of course there are a few that I have not seen (Garfield in the Rough, Babes and Bullets, Happy Birthday Garfield, and Garfield's Feline Fantasies).