Ice Punch beyond a shadow of a doubt. Swampert has a higher Attack stat, meaning Ice Punch will do more most of the time. When it's a Mega, its Attack is gigantic, further reason as to why the physical Ice Punch is better.
Unfortunately, Ice Punch is a post game move on Battle Resort. Therefore, between Ice Beam and Blizzard, use Ice Beam. It has higher accuracy and an excellent base power.
Hope I helped!