Grimrock

I think there is about 7ish gold keys in the game. The most use for the keys have been the golden locks in some secret areas scattered around, as the rewards have been the greatest (in many cases the best items of the game).

I really like the Wizard cache as,
Daniel.

With the exception of the serpent cache and to a lesser degree the archmage cache, contemplate saving your gold keys for the Vault. The items there are far better quality.

I chose the rope every time. It seems to be the most useful item from the Cache, with the orb of luminescence (or whatever it's called) coming in second.

Wizard

I'd always get the Serpents first. Now that Falling is actually incredibly dangerous in this game. Not only do you take damage. but you can actually break your parties legs, and get stuck till you drop a lot of crap.
The second would be the warriors for the fire sword if I had a rogue in the party.
The third would probably be the wizards. It has a piece of the arche mages set I think.

The rope... has been oh so useful for me. Especially when a good amount of secrets and hidden goodies have to be found by falling.
The fire sword is good as well. It deals fire damage on strikes I think. And it has a limited charge (boo) for casting fireball.
Everything else just feels worthless to me. I got two concentration for the bubble shield, and light spells. So the orb isn't useful to me. It'd just take up weight, or room in the hub... with my stock pile of crap.

Then again everything else than the mage pants are items that you can find elsewhere/can find way better for the same purpose.

Sorry where is the water temple ?

Don't take the serpent one. The gold keys are limited and you'll find another rope in the crystal mines.

So far with 74/77 secrets found my total is 10 gold keys found so far, so they are really limited resource.