What is the best way to remove the nut and compression fitting on a sink supply valve?

What is the best way to remove the nut and compression fitting on a sink supply valve?

If you're talking about the chrome compression nut and the ferrule on the copper stubout, if you can't work the ferrule off the pipe then you will have to cut the pipe off as close to the ferrule as possible. Good luck, Tom

This is an old one that has been there a long time and shows every sign of staying there? Clean next to the threads with a wire brush. Apply penetrating oil. Work it over good with a hammer, this is more about vibration than impact. Then try a wrench that fits it, not pliers or a Crescent wrench. They make special ''flare nut'' wrenches that fit around 5 sides with the 6'th side open to go over the tubing. If that doesn't do it, you can try a torch. Heat it up and then quench it with water, and again, a good wrench. Keep the torch away from things that burn. Also, you can't heat a pipe full of water.

If it still won't come off, saw off the tubing and put a 6 point socket on it.

If you are replacing a leaky old valve, consider upgrading to a ball valve. The chromed, angle stops have their place in exposed areas and save an elbow. If under the sink, use a ball valve instead, easier to use, and less likely to ever leak than globe or gate valves.

Thanks
I was able to get the ferrule off.