Unbelievably enough, someone actually did this in a controlled environment. He soaked the bullets in the solvents for given amounts of time and then weighed the bullets afterwards (already knowing the weight, 155 grain .308 Lapua Scenars), thereby establishing which solvent dissolved the most copper.
The winner? KG-12.
Now, please remember only to clean when accuracy drops off. If you shoot enough, you will know this. If you shoot a few boxes of ammo a year, don't worry about it.