Mortal Kombat gets my vote easily.
I enjoyed the first 2 Street Fighter games, but once Mortal Kombat came around with the Fatalities, Street Fighter took a backseat for me.
Not to mention that the last few MK games were quite spectacular and the latest Street Fighter 4 game was horrible, IMO. I honestly enjoy the Capcom VS games more than Street Fighter at this point.
All hail Reptile!!