In my opinion, overall the best striking art is Ba Ji Quan, "8 extremes boging", central-northern China. It's a hard, internal art.
Of course if you're talking everyday self-defense, while I stand by my choice, you need to become like a master to be able to readily apply it. And I tend to count the learning span when talking about self-defense so I'd go with i.e. something like Wing Chun.