There is no one absolute best; It
most certainly isn't Hydra-Shok or Critical Defense, either.. The cartridge that positively feeds and is most accurate from your pistol is the best choice. The ammo choice issue is made even more complex by the lack of credible research and the wealth of uninformed opinions regarding what is commonly referred to as "stopping power".
Pick one from the recommended list below. The one that you shoot most accurately, that is most reliable in the type of pistol you choose, and best suits your likely engagement scenarios.
9 mm:
Barnes XPB 115 gr JHP (copper bullet)
Federal Tactical 124 gr JHP (LE9T1)
Federal HST 124 gr +P JHP (P9HST3)
Remington Golden Saber 124gr +P JHP bonded (GSB9MMD)
Speer Gold Dot 124gr JHP
Speer Gold Dot 124gr +P JHP
Winchester Partition Gold 124gr JHP (RA91P)
Winchester Ranger-T 124gr +P JHP (RA9124TP)
Winchester Ranger Bonded 124gr +P JHP (RA9BA)
Winchester Ranger-T 127gr +P+ JHP (RA9TA)
Federal Tactical 135gr +P JHP (LE9T5)
Federal HST 147gr JHP (P9HST2)
Remington Golden Saber 147gr JHP (GS9MMC)
Speer Gold Dot 147gr JHP
Winchester Ranger-T 147gr JHP (RA9T)
Winchester 147gr bonded JHP (RA9B/Q4364)
Critical Defense.
Critical Defense provides reliable expansion every single time.
Inconsistent and unreliable performance from conventional hollow point bullets.
Critical Defense provides reliable expansion every single time.
Inconsistent and unreliable performance from conventional hollow point bullets.
Hornady Critical Defense ammunition is the pinnacle in self defense ammunition
The most effective, consistent, and reliable self-defense ammunition available today!
Don't believe the marketing BS.
The 147gr Golden Saber, Gold Dot, HST and Ranger-T all feature excellent feed profiles, excellent terminal ballistics with deep penetration equal to or better than 124gr and 127gr bullets weights, without the muzzle blast and heavy recoil commonly associated with +P+ loads