I've tested quite a few pads, and settled on the Gossamer Gear NightLight 3/4 length. This does take up a fair bit of room in your pack, but no more than anything other than the thinnest of yoga mats. It does, however, make a good staysheet for a frameless pack, and at 8oz is one of the lightest options.

It's lighter and more comfortable (softer and warmer) than the Thermarest Z-Lite, and keeps its resiliency longer. I've popped too many inflatables for them to be contenders.