Picking a knowledge.

[Ryu_Bonkosi]

A simple question, what knowledge skill is a good pick for identifying enemies weak spots if you have no idea what enemies you will be fighting?

[Urpriest]

If by "identifying enemies weak spots" you mean Knowledge Devotion, Knowledge(Local) applies to all humanoids, which means all NPC enemies.

If you mean it in a more general sense, I typically find Knowledge(Arcana) comes up a lot, especially with more inexperienced DMs, as it can be argued to apply to most anything.

[FMArthur]

Hmm... I'd usually go for Knowledge (Local) to get all the Humanoids. It's hard to have a campaign totally lacking humanoids, unlike other creature types.

[Kelb_Panthera]

Actually, each of the knowledges is used for a certain set of creature types. I'm gonna put in a vote for Knowledge (local) or Knowledge (the planes) since they both cover a fairly large group of creatures.

[Worlok]

Assuming you're talking about D&D 3.5, Knowledge (nature) covers the greatest variety of enemy types. Add Knowledge (religion) if you suspect undead, Knowledge (local) in civilised areas with a lot of humanoids, Knowledge (the planes) in case of outsider or elemental presence, Knowledge (dungeoneering) when below ground level and at a risk of running into oozes or aberrations and perhaps Knowledge (arcana) if you suspect dragon involvement, construct use or a population of magical beasts.

[Zaq]

The biggest ones are Nature, The Planes, and Arcana, I believe in that order. After that, it's a toss-up between Local, Religion, and Dungeoneering. Architecture and Engineering comes up occasionally, but generally not too often.

[SwordChucks]

You may be able to convince your DM that ranks in Heal would give you a better understanding of where to strike, maybe as a synergy bonus.

[Corwin_of_Amber]

Under skills in th PHB it shows what is identified by each. I haven't played a Knowledge Devotion character yet, but in retrospect Arcana, Local, Nature, and The Planes would be the most useful. Dungeoneering and Religion are rather limited in application in both number of creatures aplicable and out of cambat use as well.

Arcana (constructs, dragons, magical beasts)
Dungeoneering (aberrations, oozes)
Local (humanoids)
Nature (animals, fey, giants, monstrous humanoids, plants, vermin)
Religion (undead)
The Planes (outsiders, elementals)

[Psyren]

What class are you? Many PrCs have knowledge (X) requirements, so you can synergize by focusing on the one(s) that benefit your build directly.

Similarly, you should pick based on your party members as well. If there is a cleric, leave Religion to them as they have probably invested in it. If there's a Druid/Ranger, they should have Nature covered. Arcanists will of course have Arcana, and the Rogue/Bard should have Local.

Also, is it just me or do Knowledge (Local) and Gather Information seem redundant?

[grimbold]

does knowledge even work that way?

[Curmudgeon]

does knowledge even work that way?

There are 6 different Knowledge skills relating to the different creature types that exist in D&D.

In many cases, you can use this skill to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster’s HD. A successful check allows you to remember a bit of useful information about that monster.

For every 5 points by which your check result exceeds the DC, you recall another piece of useful information.

It works exactly that way.

[Hanuman]

Check skill synergy, clerics need religion, if you have spellcraft then get arcana, nobility and royalty f you're a face, and anything else I'd recommend geography as "avoiding natural hazards" is always a good argument.

[Psyren]

does knowledge even work that way?

Not sure what you mean: you have to pick the right skill to identify the right creatures, that's how it works. It was Neverwinter Nights that lumped them all into "Lore" and didn't even use them for identifying critters.