Disclaimer
I have not even read all the creatures, let alone used them, so my evaluation is based purely on what I consider good monster / NPC design:

1. Monsters should be usable in many games (not too setting / locaion specific, not too low or high CR, isn't based on an absurd concept (like the creature is the Sun), running the monster isn't very time consuming or requiring a lot of bookkeeping...),
2. Monster's narrative concept should be interesting and somewhat unique
3. Monster's mechanical concept should be interesting and represent what makes it narratively special (not just high AC, good bonus to hit, does tons of damage, etc)

Scarred Lands Creature Collection is a great bestiary and its FGU implementation is solid as well. It has good number of monsters, most of the monsters I have read fulfill my criteria for a good moster desing, the art is good quality and not a single monster has felt stupid, completely boring or unusable. So far my favourite bestiary has been Tome of Beasts but Scarred Lands Createure Collection might just take that title after I have time to use its monsters more.