If you can cut out the copper elbow and fitting, on the next one you can try this method:
1) Use a small amount of pipe dope on the male threads prior to applying no less than 3 full wraps with Teflon tape.
2) For Teflon tape, they make some varieties that are 'extra thick', which help compress to fill the gaps.
3) Before assembly, another thin application of pipe dope (Rectorseal 5 or similar) on top of the Teflon can help seal up the threads of particularly pernicious leaks.
It may also be worth confirming that the taper of the threads on your valve and your fitting match.
If you cannot or do not wish to remove the fittings or valve, the scorched-earth approach is to use a two-part epoxy putty. Mix the stuff and apply it firmly around the entire threaded area and fitting. The fitting should be bone dry. For extra strength on 'field-expedient' fixes using 2-part epoxies, I've included a small strip of fiberglass tape to embed between two small layers of the putty. It's not necessary, but it might help enhance the strength/longevity of the repair.