Good old Hoppes No. 9 for general cleaning. It is not a super strong solvent but the key is to let it work by giving it some dwell time and not be in a big hurry. For barrels that have a lot of copper fouling, I will use Sweet's 7.62 solvent. It has a lot of ammonia and needs to be used in a well ventilated area. The manufacturers of most of the copper specific solvents don't recommend leaving the bore wet for more than about 15 minutes.

Hoppes No. 9 can be left to soak in the bores for days with no ill effects. Ballistol works as a light cleaner, and MPro7 does a good job on loosening carbon buildup on cylinder faces.

If I had to limit myself to one solvent product, I could use Hoppes No. 9 and just be prepared to let it work which it eventually will. And if I had a Nickel plated gun I would not let the Hoppes sit on the finish for more than a minute or so.