Montana extreme IMO puts the rest in the back seat.
+100
Montana Extreme Copper Killer tops the list of Sweets(that I used for 10+ years),Butches, Barnes CR-10,and Shooter's Choice amonia based products.
Bore tec eliminator and K-g 12 are both great non amonia based solvents, not quite as effective as Montana Extreme Copper Killer, however.
I have yet to try the product that gun slick makes.
I shoot a lot of guns, factory and custom frequently no matter what the weather out of a heated/AC house. It does not take long to see if a product is a will remove the copper quickly with little effort if you are shooting a bullet with a lot of bearing surface or very fast velocity.
Lots of products will remove copper with multiple applications, but Montana Extreme Copper Killer is in a class by itself(oil based also). M Extreme Copper Killer is an amonia based product. If you use one of their plastic brushes with the Copper Killer, you will have a one shot deal on removing coppper unless you are shooting an old Military rifle.
If you shoot a Savage, old military rifle, or an old Ruger bolt or #1, you are in a special class of needing the Montana Extreme Copper Killer because MOST of these rifles really copper up in a hurry.