I believe it is what you are drawn to personally (if you are drawn to anything at all.)
Just to give you an example from myself, I felt a strong connection to the golden obsidian spheres at the store where I work. I keep one on the table to absorb and cleanse any negative energy that a sitter might bring in.
Next to the obsidian I keep a raw amethyst point and a raw citrine point. I don't have a problem with tumbled stones, but I feel that the raw versions have more absorptive power, and the points can act as a sort of projector, recycling the energy for psychic enhancement from the amethyst, a playful energy from the citrine, and cleansing from both.
As some others have said, lapis lazuli is great for psychic enhancement. But, not to sound shallow, I don't like how it looks with my other stones. I do keep some in the bag with my obsidian, so I feel it can absorb some of the energy that way.
I'm certainly no expert on stones and crystals, but I believe that their power is based on your own beliefs and whatever connection you feel to certain pieces.