9mm, most popular military round & a lot of LEA
40, probably taken the majority of the LEA contracts
45, very American, still popular w/ elite LEA & competitors
If I had to bet on ammo availability, it would be in that order. Many 45acp shooters reload, a few 40 & not that many 9mm.
"Given adequate penetration, a larger diameter bullet will have an edge in wounding effectiveness. It will damage a blood vessel the smaller projectile barely misses. The larger permanent cavity may lead to faster blood loss. Although such an edge clearly exists, its significance cannot be quantified".