Tiny dogs, only because 99.9999% of owners of tiny dogs are idiots who don't understand it is still a dog, so the animal becomes unruly, untrained, and often dangerous. I respect an owner of a tiny dog who treats the thing correctly, keeps it properly leashed, socialized, and behaved, but I've met a total of two people like that in my entire life. And I say that as someone who's grown up in a family that raises and breeds AKC dogs. Been around many many dogs in my life.
Chihuahuas are the worst. I'd genuinely be happy if they all just died off, the breed should never have existed.