There are different qualities that make up a good cooling mattress topper. For the most breathable and air-flowy materials, I'd recommend a latex topper like PlushBeds or Avocado. There are also cooling toppers like the one from GhostBed that use special phase-change materials, and they can be very effective. You should also look for materials you find comfortable. If you like a fluffy, soft feel, the Brooklinen Down Alternative topper would be a cozy choice.