For the Proficiency of the week, I have selected the non-weapon proficiency Biology, which costs one slot, and as a general proficiency, is available to all characters at that one slot cost. It uses Intelligence as its base, and has a default -2.
Here is the core-rules description:
The character has a basic knowledge of naturalistic biology. The character has a concept of cells and ‘germs’, understands the basic organization of animal and plant life from the cellular level to the organ level (but does not understand the concept of organelles or anything requiring a high-powered microscope. Most importantly, the character has made a study of the natural (and supernatural) creatures and plant life of their homeland, and can make a proficiency check to know about creatures the party encounters. For example, Frank, a druid, has a proficiency in biology. When his party is attacked by a hydra, he makes a check and accurately identifies the subspecies, warning the party’s tank that if he cuts off a head, two will grow in its place.
The most frequent use of the biology proficiency is to find out if a creature is natural or unnatural, or to find out if a creature is typical to an area. A biology proficiency can also provide general information about an area from the obscure "Something is wrong with the wildlife here," to the specific "It looks like something has poisoned the wildlife here." It can also be used to determine if something is poisonous or edible, sometimes with a negative.
Creatures that are completely magical or extraplanar, or are created from parts of other creatures will usually fail to have information from a biology check, which can provide information in and of itself.