Freestyle and backstroke are perhaps the two least tiring strokes when performed correctly. Because there’s no timing or head turn requirements for breathing during backstroke, it could be considered an easier starting point. Freestyle can worry new swimmers because you need to put your face in the water to perform it well. Depending on fitness levels and starting point, breaststroke might appear simple but when done well it’s actually highly technical. My advice would be that confidence, the ability to relax, and timing of the breath should be early aims regardless of stroke.