The best gun cleaning solvent can depend on personal preference and the specific type of firearm. Some popular options include:

1. Hoppe's No. 9: A classic choice known for its effective removal of carbon, lead, and copper fouling.
2. Break-Free CLP: A cleaner, lubricant, and protectant in one, ideal for overall maintenance.
3. Ballistol: A versatile product that cleans and protects, safe for wood and leather as well.
4. M-Pro 7 Gun Cleaner: Non-toxic and biodegradable, effective for removing fouling without harsh chemicals.
5. Kleen-Bore: Good for a variety of firearms and offers a strong cleaning action.

It's a good idea to choose a solvent that is compatible with your specific firearm materials and to follow the manufacturer's recommendations.

Choosing one depends on your specific needs and firearm type!