Roundup works great. I used to get a good case of poison ivy twice a year, growing up on a farm. Now that I can spot the plant from ten feet while on the move, I never get it. Several times I have needed to weed the vine out from an area of ground cover, where it was not practical to spray. By covering up with long pants, long sleeves, and gloves, I could pull it out safely.
Cortisone creams offer the best hope of any topical preparation.
For more severe cases, especially involving face, hands and genitals, genuine relief comes with prednisone tablets or a cortisone shot. These can reverse the rash fairly quickly, and are fairly safe unless given too many times a year. Your doctor will help decide if you need these stronger treatments. Ten to fourteen days is a more appropriate treatment, starting at 60 mg and tapering slowly. A final note - many people ask about the shot to prevent poison ivy.