Do you have a fondness for any sort(s) of enemy? Also, the the others seems to have more exotic minsters, plus whatever races were recently focused on.
Bestiary 2 is really good.
I have 1,2 and 3 and found three the most fun.
I also saw someone ask a similar question to this on Reddit where on the 5, 3 was most often top listing but 4 was also pretty popular as I recall.
They each have their own flavor to some extent, some more oriental some more horror, some more Sci-Fi, it might be an idea for you to outline which style of monster most tickles your fancy.
1, 2 and 3 are pretty cool, although 3 is more focused on Eastern myths, some people don't like that flavor... 4 is pretty "meh" IMO... Mostly because Paizo turned it into a paid advertisement for the then recent Mythic rules. 5 is okay.
I think 4 is alleged to have a lot of Horror style stuff in it and 5 went a bit more sci-fi and nightmareish.
3 is more eastern style.
1 and 2 are the most classical fantasy.
Between my son and I, we own all 5. My faves, and the ones I own, are the 1st, 2nd and 5th. My son bought 3 & 4.