couldn't be more different

built different

Able to do things which others cannot; extremely talented or unusual.

couldn't care less

To not care at all; to have no interest or concern; to be apathetic.

couldn't carry a note in a bucket

Not musically talented, unable to sing.

couldn't happen to a nicer

Sarcastically asserts that those in question thoroughly deserve their fate.

couldn't organise a piss-up in a brewery

Is unable even to organise a project that requires no planning; is incompetent; used to describe someone as greatly incompetent.

couldn't stop a pig in a passage

To be bow-legged.

different breed of cat

Something entirely different; a whole other thing or matter.

different drummer

Someone or something that implies individuality, originality, or nonconformity.