For me it would be 1989. I love most of the songs on it, but it's the only album with songs that I don't actively seek out to listen to. If they come on on shuffle, I'll absolutely jam out to HYGTG or AYHTDWS, but I don't go searching for them.
I think it's honestly just the sound on 1989 that makes it my bottom album. There are some amazing songs on it, don't get me wrong, but it's the only album that I feel kinda "eh" about as a whole.