Depends on the fight. If it's a BG/Arena, then Feral should win. DKs are THE easiest class to kite and Ferals are one of THE best kiters. Put 2 and 2 together.
Stack the dots, do the damage. Low on HP? Stun/Vortex/Root/Cyclone DK and Displacer Beast/Stampeding Roar/Dash out and heal. If the DK has Death Pact, they could only heal every 2m and if they have Conversion it'll be even easier because they won't have RP to heal themselves.
DoT/kite and the DK will die breaking his keyboard in rage.
The only time a good DK will beat a good Feral are in duels where the Ferals aren't as free to kite.