Midship, and as low as you can go, deck 1 or 2. Simple physics, just think of the ship as a bike wheel cut in half to make a half circle. The closer you are to the axle, and the bottom, the less motion you feel.
Having said that, we stayed aft on deck 7, and felt very little motion except the night we cruised to Castaway Cay. Very rough, but none of us gets sea sick. I've been cruising for 27 years, and ships now have stabilzers that really smooth things out. My first cruise was on a ship about 1/3 size of the Magic (The Mermoz), without stabilizers. 14 days Eastern Caribbean, there were some nights it was so rough, you were walking up hill one minute down the hall, and then running downhill while walking the same direction.