* Irreplaceable: This is the most common and straightforward option. It directly means "unable to be replaced."
* Unique: This word implies that something is one of a kind and therefore cannot be replaced.
* Invaluable: While not directly meaning "cannot be replaced," it emphasizes the high value of something and suggests that replacing it would be extremely difficult or impossible.
* Priceless: Similar to "invaluable," this word emphasizes the high value of something and implies that it cannot be replaced with something of equal worth.
The best word depends on the specific context and the type of thing being described. For example, "irreplaceable" might be the best choice for a lost family photo, while "unique" might be better for a rare antique.