Strongest:
* Incorrigible: This implies a stubborn refusal to be corrected or improved, often with a negative connotation.
* Insatiable: This suggests an endless desire that cannot be satisfied, leading to constant dissatisfaction.
More general:
* Unpleasable: This is the most straightforward, directly meaning "not able to be pleased."
* Disagreeable: This indicates a general dislike or aversion to things, making it difficult to please.
* Capricious: This implies unpredictable and whimsical preferences, making it hard to know what will please them.
More specific:
* Fastidious: This suggests a strong preference for perfection and a high standard that is rarely met.
* Cantankerous: This describes a person who is grumpy, argumentative, and difficult to deal with.
* Unresponsive: This highlights a lack of reaction to attempts to please, suggesting indifference.
The best word to use depends on the specific context and the nuance you want to convey.