Usually if I don't get into a book, I just stop reading it. I don't know that I could list a "least favorite" because I just don't finish them.
I've tried reading the Pern series twice, and L.E. Modesett's books twice, just off the top of my head and I can't get into them.
Pern's books just don't raise my interest quick enough and Modesett's just don't plain make sense to me. His writing style is a bit off.
But then again, I used to hate the Gunslinger and I picked that one up again recently, and I find myself reading the whole series and looking for other books King used to reference the Dark Tower (It).
There are also some authors who I enjoyed some of their stuff, but then I moved on when they did other things.
Terry Brooks is a good example. I loved his early Shannara books and his Landover books, but his Word and Void series was nothing I was interested in (probably because of the modern time setting) and his later Shannara books after Antrax didn't really appeal to me. I think part of it is the way he started adding airships and things like that (anything to do with boats, or flying usually doesn't catch my interest) and partly because the series didn't go the way I felt it should in regards to Walker Boh. I've never been a fan of song music either. (The wishsong) it just sounds silly.
Raymond E. Feist is another one. I loved his Sethanon related stories, and read his stuff beyond that up until he got to like the 3rd generation of his characters and then I just couldn't keep interest anymore. He'd killed off all the characters I enjoyed originally and his new one wasn't good enough.
Mathius