In Gloomhaven, is the leading card chosen for initiative after or before monster cards are revealed
The rule is a bit ambiguous, but the leading card is selected in card selection and only revealed in initiative phase.

Rules:(emphasis theirs)

Select cards:


  Of the two cards, one should be selected as the leading card, which will determine the player's order of imitative for the round


Determine initiative:


  After players have either selected their two action cards or declared a long rest, the players reveal a monster ability card for each type of monster that has at least one figure currently on the map. In addition, each player not taking a long rest reveals his or her leading card for the round. 


This means when you pick which two cards to play, you also decide which card will be leading at that time, then player leading and monster cards are revealed at the same time.