Of the core skills, including non-core uses:
Concentration
Diplomacy
Use Magic Device
Tumble
..............
Spellcraft
Intimidate (terrible w/o splat material to up duration, lower action cost, make it an AOE, etc...)
Bluff
Handle Animal
Balance (can prevent flatfooted, resist trip, and Swift action stand up w/o AoO)
Knowledge (if you have to do them individually, Nature is best as it covers the most)
Dotted line to separate them into tiers, as IMO those above the line are a significant step up from those below, which are also good.
Of the noncore stuff I'm familiar with, Iaijutsu Focus, Autohypnosis, and Use Psionic Device and Psicraft (which I'd just put alongside their magical counterparts) would go on my list and knock off some of the skills at the bottom of my list. I'm not familiar with Lucid Dreaming, but from what I've heard, it's the most stupidly broken skill ever.