Favorite or favori !

Stephen 

It is ‘ mon activité favorite …’ . L’activité - feminine noun, hence feminine adjectival agreement.

For completeness, the fact that ‘ le ski ‘ is masculine has no impact on the adjectival form here.

See attached lesson for usage of possessive adjectives immediately preceding nouns or other adjectives commencing with vowels or h mute.

https://www.wordreference.com/fren/activit%C3%A9 

 "Ma" becomes "mon" with feminine nouns starting with a vowel or mute h (French Possessive Adjectives)