In terms of aesthetics, I've always preferred Street Fighter's bold and rich graphical design to MK's ghoulish and grainy style. There is such a huge variety to the characters and stages that it never gets old, and while the art style has evolved over the years each incarnation remains timeless. Gameplay-wise, admittedly I haven't played much of MK since the original edition but for me there isn't any fighting game that can compete with the sense of satisfaction that comes from playing a good session of Street Fighter. I think it's underrated for the amount of effort that goes into making the characters balanced while maintaining lots of scope for combos and strategy, and also the depth of variety between different characters where even normal punches and kicks are unique for each fighter.