Sucker Punch does damage and is an increased priority move. If the target does not use an increased priority move during the round that it is used, Sucker Punch will go first regardless of the user's or target's Speed. If the target also uses an increased priority move during the same round that the user uses Sucker Punch, the attack order of the users will be determined normally.
Quoted from Bulbapedia, here
So basically priority moves will go before it.