Yep. Nine millimeter. It's accurate, powerful, effective, versatile! You can have a pistol chambered for it to fit in the palm of your hand, or a service sized gun to fit 16 or more rounds. As Buck said, there are lots of 9mm rounds available, in various weights and configurations. They can be had cheap. Like $10 a box cheap. I like nine millimeter. Not saying it's the best, but it might be, with all the plusses.
And yes, it's almost to the point where you can find 9mm ammo anywhere. I expect it to show up at gas stations beside the donuts and coffee sometime. Maybe they'll offer it to you at McDonald's soon. "Sir, would you like a box of 115 grain hollow points with your coffee and egg mcmuffin? They're only $8.99 a box today."
Most abundant depends upon your location. Assuming you are in the USA the most abundant pistol cartridges are: .22 lr, 9mm, .38 special. .357 magnum, followed up by .40 S&W (and sometimes .45 ACP so it deserves a place on the list but gets a asterisk).
When I teach firearm safety courses I often get asked this question. The answer is anything you can find walking into any big box store, sporting goods store or gun shop. The five (six*) calibers I've listed you can find anywhere they sell ammunition.
As an added guide think of any caliber that the local police or military uses too. Hence, the reason for .40 S&W on the list. Yes, some federal agencies seem to have a love affair with .357 sig but you can't find that all the time at big box stores. The .45 ACP is still very plentiful but its discontinuation by many LE and military agencies have diminished its popularity slightly. The .45 ACP will be around for years to come but in a zombie apocalypse don't plan on finding .45 ACP in an abandoned police station or military base.
That said, if you are going for a revolver get a .357 magnum (so you can also shoot .38 special) and if you are going for a semi-automatic pistol then go with the 9mm.
One additional comment: with a revolver you never have to worry if your pistol will digest the ammo nicely, have failure to feed problems or have a stove pipe (ejection issue). I have a revolver chambered for every semi-auto pistol that I have. This way if the semi-auto barfs on a particular brand of ammo it goes into the revolver. No regrets trying out a new brand of ammo.
@Veli Hopea Good find but that table may be a little misleading since it is from 2015 and from one retailer. At that time retailers were basically selling anything and everything they had on hand so stock availability drove the sales.
Additionally Lucky Gunner tends to sell to specific clientele base further skewing the results. What I am saying that bulk sellers like lucky gunner tend to have customers interested in shooting competitions (yes, a generalization) and buy bulk ammo for practice. Hence the large .45 ACP numbers since that is a hot item among competition shooters.
A more accurate measure would be production figures across multiple US ammo manufacturers and importers cross referenced with multiple ammo sellers figuring in everything from bulk sellers down to the local Wally-Mart.
Ballistic gel? I like to see my steel plates move when struck as opposed to just ring. 9mm does not hit my plates hard enough. The 45 gets the plate swinging telling me there is a significant amount of energy transfer on contact. Granted these are all FMJ. I have also seen demos with pig skulls and the 9 vs 45. Results are much different then gel.
The 9mm depends on the special new HP designs. Those rounds are not out in the public en masse. the bulk of 9MM is FMJ not custome defense rounds so it is a moot point.