Tough one, some type of fly which tends to be pretty worthless unless you are a novice or intend to use a lot of drugs to get something out of a pretty poor stimulus.
The body is a system. It is best stressed as a system. It adapts best when treated as a system. The reason all the compound exercises are the big mass builders is because they allow the system to move a lot of weight through a basic range of motion. They load and stimulate the system. If isolation was in any way as good as most BBers seem to think it is, all those "curl boys" would have nice big arms yet it is the guys in the rack squatting and pulling who do a few sets of arm work a week who have guns. Isolation has a place but people would have a much easier time adding muscle if they dropped 80% of their crap exercises and just worked hard at improving the big blocks over a period of time. Maybe read that again.