The Euro Death Knot (overhand), Double Fisherman's or ring bend (water knot) are tested by Black Diamond to see which one is stronger. The Double Fisherman and water knot are the strongest, but the E.D.K is certainly strong enough for rapping as their QC guy explains:
“The reality is that all three of the methods for joining two ropes for rappelling that I tested were PLENTY strong for the forces seen during a typical rappel (i.e., bodyweight-plus, taking into account some shock loading when bouncing around, jiggering with tangled lines, not smooth rappelling technique, etc).
For what it's worth, when I started climbing I always used the Double Fisherman's, but now I've fully converted to the Euro Death Knot—it's fast to tie, plenty strong, less likely to get hung up when pulling and easy to untie. And finally, no matter what rap knot I tie, I always leave long tails (like at least 12 inches.)