Does what it says on the tin, really.
Do you hate the Pertwee era and everything it stands for? Have you always loved the Slitheen? Do you think that calling people names and swearing at them is an intelligent way to have a discussion? Do you wish Peter Capaldi hadn't been cast? Is there a popular writer than you just can't stand?
Personally speaking, I love Love & Monsters, truly, unashamedly, and unabashedly. I think it's brilliant, and I've enjoyed it every time I've watched it. The characters are, I feel, quite well realised, and it has a rather fascinating look at the effects of the Doctor. And, obviously, it's a rather effective metaphor for fandom, isn't it? (Well, not really a metaphor.)
So! What's your unpopular opinion? And, of course, in the interests of discussion, you've got to be ready and able to explain why.