I voted for SF, but they both personally hold a special place in my heart. HOWEVER, I would have to say that Street Figther II: The World Warrior is by far the best fighting game out there. I heart Blanka.
Ok I Love Both Series But I Would Have To Go With Street Fighter 4.
Reason: Street Fighter Has More Depth and Has Alot Of Replay Value and It Takes Time For You To Learn Their Moves and Adapting to Other People's Style As Well. Mortal Kombat has always been fun for a little but with Street Fighter 4 out the depth in the game is insane allowing you to string together combos that you didnt even know you could.