Formal:
* Fastidious: Excessively concerned with detail and accuracy.
* Discerning: Showing good judgment and taste.
* Critical: Finding fault easily; not easily satisfied.
* Demanding: Requiring a lot from others; difficult to please.
* Captious: Finding fault unnecessarily; tending to criticize.
Informal:
* Picky: Very careful about choices and details.
* Finicky: Difficult to please, especially in matters of food.
* High-maintenance: Requiring a lot of attention and care.
* Hard to please: A straightforward way to say someone isn't easily satisfied.
* Difficult: A general term for someone who is hard to work with or please.
Other options:
* Unsatisfied: Not content or happy with something.
* Discontented: Not satisfied or happy; feeling restless or dissatisfied.
* Unimpressed: Not impressed or influenced by something.
The best word choice will depend on the specific context and the tone you want to create.