General synonyms:
* Style
* Panache
* Charm
* Sophistication
* Dazzle
* Pizzazz
* Spark
* Verve
* Liveliness
* Vibrancy
* Showmanship
* Eloquence
More specific synonyms:
* Talent (if referring to a natural ability)
* Skill (if referring to a developed ability)
* Taste (if referring to aesthetic sense)
* Imagination (if referring to creativity)
* Originality (if referring to uniqueness)
* Presence (if referring to a commanding presence)
* Personality (if referring to a distinct character)
Antonyms:
* Blandness
* Dullness
* Mediocrity
* Clumsiness
* Stiffness
The best synonym for "flair" will depend on the specific context in which you are using it.