Oh, no. Another 'best ammo' thread.

I'd encourage you to read the AR15.com Ammo FAQ article. It discusses both handgun and rifle self defense loads.

While you're right to ask the question about what's the most theoretically effective round, don't lose sight of practical aspects as mentioned in the above posts. Shot placement, and reliability of the ammo in your gun is paramount. If you can't hit what you aim for, or the round fails to feed, extract or ignite, you're screwed.

Read the above FAQ.
Buy a few boxes of different recommended types.
Go to the range and practice with them.
Make a choice and be confident with it.

Ignore cost. Ammo is cheap. Life is expensive.

HTH.