Titegroup, 231/HP-38, Bullseye, Universal. Buy whichever's available.
As you gain reloading experience you'll quickly learn that there is no "best" powder. I could have listed 20+ powders but your limiting factors are usually price, availability, and versatility. That's why I really like Titegroup. It's cheap, does almost everything well, and readily available (usually). Personally, I don't like really slow powders for 9mm, so Universal or there abouts is my cut-off. Some have mentioned HS-6, and of course it works, but I would never load something that slow. It would take in the area of 6-7 gr of HS-6 to accomplish what Titegroup will achieve with 4-5 gr. Again, personal preference.