Synonyms for Priceless:
General:
* Invaluable
* Precious
* Valuable
* Costly
* Worthless (ironically)
* Beyond price
* Beyond compare
More specific:
* Unsurpassable
* Unparalleled
* Unique
* Rare
* Treasured
* Highly valued
* Irreplaceable
* Immaculate
* Exquisite
Synonyms for Matchless:
General:
* Peerless
* Unrivaled
* Unbeatable
* Unmatched
* Unequalled
* Supreme
* Superior
* Outstanding
More specific:
* Exceptional
* Remarkable
* Extraordinary
* Phenomenal
* Uncommon
* Incomparable
* Singular
* Perfect
Figurative:
* One of a kind
* A cut above
* In a class of its own
* Head and shoulders above
Remember that the best synonym will depend on the specific context of your writing. Consider the specific nuance you want to convey and choose a synonym that fits the overall tone and meaning.