Yeah, like everyone else I will recommend the APG. Ultimate Magic and Ultimate combat have alot of good stuff in them but the APG is just amazing.
As for the Bestiaries 1 has the more common creatures. I personally prefer 2 slightly over 3. 2 contains alot of creatures of the Outsider type while 3 contains lots of creatures from real world mythology - largely but not solely asian.
Paizo gave away a book on free RPG day a couple of years ago just before the PFRPG was released. It is called the Bonus Bestiary and it contains around a dozen creatures I believe. All of these were eventually re-released in Bestiary 3 but until you get that it is nice to have especially considering it is available as a free PDF download here.
And then since Paizo is so big on open content all of the creatures from B1-3 (as well as the material from the APG, Ultimate Magic, Ultimate Combat, and more) is available online here. You can try it before you buy it.
This same material, as well as alot of 3rd party material, is available on a fan-maintained site here.
