In replacing the hot and cold water supply valves under the sink I have a slow leak. The end of the copper pipe has a ridge on it with a nut receptacle behind it where I am screwing the new valve into. I have used tape and grease with no luck. Any suggestions?

That "ridge" is called a ferule and that compresses against the nut and the pipe for a watertight connection. No grease, pipe dope or tape on the threads is necessary. Crank down hard on the large nut and see if the leak doesn't stop. If that don't get it then you might have to cut the ferule out and replace it. Good luck, Tom

Or... take all the tape and grease off the threads and dry the connections and apply a small amount of pipe thread compound on the ridge [ferrule]. If you overtighten the brass nut it will strip. If not tight enough it will blow off a new connection or cause an old connection to leak.