V12, of course, preferably as large as possible. 
“Best” is highly subjective and dependent on the situation. The duffel bag holds the most, for instance, but the encumbrance on it is HUGE. Dropping worn bags when entering melee can range from “always a good idea” to “ABSOLUTELY REQUIRED OR YOU WILL DIE”, depending on the situation.
The problem is encumbrance - torso encumbrance will make it harder for you to hit (which can be offset by high skill and weapons with bonuses to hit) AND slow you down by making your melee attacks take more time (I don’t know of anything that helps with that, so even on high level characters, it can still matter).
With decent dex and a weapon with a hit bonus, you can reliably kill regular zombies even with CRAZY high encumbrance, but go up against something that’s even slightly harder to hit (zombie child, skeletal dog), and a high encumbrance means you’ll NEVER hit them. Mi-go, giant wasp, manhack? Yeah, not going to happen.
Generally, I try to keep torso encumbrance under 30 for melee combat, even for pretty skilled characters. I’ll carry a duffel bag for bringing the loot home, but if there’s something other than a regular zombie in my way, the duffel bag comes OFF.