What is the worst mix breed combination dog?

Rottweiler and wolverine.

Anything mixed with pitbull

Pittbull and rottweiler

As far as I'm concerned there is NO mix-breed dog that's good, or bad. Messing around with long established breeds is what BYBs do - for money and the sooner the demand stops, so should the supply. This kind of irresponsible breeding is building up way more problems than ever purebred breeding has done. BYBs don't test, or bother about the temperament of these puppies they are producing because it is all about income.

Presa Canario and Akita.
that should be lethal

Caucasian Ovcharka and Micro-mini Teacup Chihuahua. lol

Working breeds that have been mixed with toy/companion breeds. The Pomsky ( Pomeranian, Siberian Husky) breed comes to mind.