This is the easiest question to answer.... but has to be answered by you!
The absolute best pistol for [personal] defense (prefer a 12 guage for HD) is the one that you are most comforatble with and shoot the best with. My carry handgun is not my favoite pistol that I own... In fact it is not even close to my favorite, but it is reliable and the one that can get 2 shots to center of mass the fastest. Every single gun made has a fanbase for different but important reasons for each owner.
It makes no difference what brand or caliber I use because my choice will likely not translate to you. For example .45 acp is the cheapest ammo for me because I reload a ton of it.
Nearly any handgun 9mm/.38spl and bigger that feels good in his hand and is made from any of the major gunmakers. New, or used (not abused) will be a great choice!!
Plastic frame? Any Glock, XD, FN, M&P, Walther, Beretta, Sig Pro, HK, or Ruger will be a great choice, if YOU like the way it feels, and are comforatble with the controls.
Steel/alloy frame? CZ, Sig, Browning, FN, EAA, Beretta, 1911... Lots of GREAT new and used choices.
Revolver? Tons of GREAT choices in the used market for WAY less than your budget... Both Ruger and S&W are solid choices. You could buy a nice police trade in (model 64, 65, 10, 13.... ) AND an 870 shotgun for around 600.
I would just go to a gunshop & fondle guns until I found the one I like the best.... Remember though, your tastes, needs and wants will change over time. That is why most of us have a lot more that one... they are all good.