In my case, it is almost impossible to assume reliability and availability, so please allow me to post without this assumption.
Reliability is the most important factor.
If you live in a place where to get ammo is a real problem, and you might find ammo from WW I or WWII, you have to be happy if you get ammo that works.
Old ammo works good in a HP (blessings to Mr. Browning), but might not work in new gun models or brands.
So, sometimes, depending on where you are located and depending on the critical situation, you have to trust not only in your ammo, but in the ammo-gun combination.
Following this forums instructions, I keep some good, new Federal Hydra shock for an eventual bad situation, (because of penetration, expansion, accuracy, etc, etc) but after the ammo is gone.... combination of gun ammo reliability is the most important factor.
Once my reloading machine is here, and I am able to make my own ammo, and not depend on what I can get, I guess a good combination of penetration and expansion are going to be my carry ammo goals.
So, my answer assuming reliability & availability as a given, when I will be able to make my own ammo is a good combination of penetration and expansion. (Assuming you will not change my mind after the questions about reloading I will ask you when the reloading machine is in my hands.)