Without a doubt, the best and most useful skill in the game, that is also one of the easiest to raise to ridiculous levels, is....
....Diplomacy!
Why is that?
With this skill you do not need weapons or offensive spells anymore.
Combat encounters can be prevented.
Ugly, nasty, creepy things, best left forgotten, can now call you daddy.
And you can pet them too.
Nothing can beat that.
I thought this skill so broken (since I love hack'n slash, and my wife doesn't) i now make opposed checks at all instances, to let my bad babies of doom, get a chance (however slight) to actually fight the heroes, instead of just cuddling!
Bluff is nice, but it's really much better to talk the truth and convince everyone that it is THE way everyone should think. You get evil guys to rethink their evil ways (or good guys, if you happen to be evil), and whether you believe it or not, you do not actually have to kill stuff to get better at Craft (landscape gardening) and Profession (butler).
This single skill can ruin your game if you, like me, want to use miniatures, floorplans, hours of tactical arguments and rules discussions, small pieces of paper as winged random monster counters you didn't pre-planned for, instead of actually playing a role (like for instance, my wife likes to do).
She: "I want to talk to the creatures, not kill them!"
Me: "But you are a holy slayer, and they are pit fiends?"
She: "Well, I use my Diplomacy to tell them to come live with me, while I re-institutionalize them as actual living persons with emotions and needs, make them see things differently, back at my castle where all those other, now so much nicer, friends* live."
*)Like for instance a kyton, a succubus, a half-demon, an ancient black dragon, a mature red dragon polymorphed into a leopard kitten, a baby hydra, a marilith converted to organist, at least five vampires by now, five black dragon hatchlings, a pack of former Malar-worshiping werewolves now into flowers and nature preservation, a harpy, an ex-Blackguard of Beshaba, a former lich now transformed into flesh, and lots of drows (since they are sexy) all living together with one angel cohort.
So the final word on my tombstone, my ephitath, will read:
"Diplomacy got the better of me"
A Final Note: I've becoming an expert ambusher since this skill was brought up, since surprise attacks doesn't allow room for talking.
And the castle is full of creatures, as it is. No more room for any more aberrations.