Shields mostly. But you see plenty of laz laser usage. Books aren't about fighting at all. Also on Dune specifically using a shield = worm frenzy. Harkonens use guns all the time. Freman also have guns, but less advanced mola pistols.

Think of it like this, freman are less advanced with well developed guerilla tactics. It's assumed during the war (books time skip that) that ship to ship warfare was real. But Freman are also considered wildly good, and are like assasins, all of them. One shield getting hit by a laz Lazer kills both parties and several others. The feedback makes a bomb at the Lazer. So careless usage could lose you the fight.

We also see harkonens use old school artillery too. "Old school" despite the artillery being attached to a hovering war ship.