Theoretically, the fastest and hardest bullet has the best chance. For example, a 135 gr at 1,600+ fps. While a IIIA would almost certainly stop that, a IIA might not.
9x25mm Dillon would give you a way better chance of penetrating soft armor but your best option with a hand gun is going to be training for failure to stop. There are many reasons for which one or more hits to the chest might not stop a threat and all of them are remedied by a hit to the CNS....
....except for unstoppable robot killers from the future. The only thing that stops them is a phased plasma rifle in the 40 watt range.
Just here for the 10mm death ray talk.