I just replaced 18 year old shut off valves under sink with newer style ones. I replaced the old nut and ferrule as well on copper pipe. I am getting a leak between nut and copper pipe.
I have tightened nut quite a bit and still keep getting drips.

You should have kept the old compression nut and ferrule and installed the new angle stop on it. Do you have enough room to cut the stubout back and install a new ferrule? Cheers, Tom