I have spent an embarrassing amount on air pads. I like self inflating style. I started with some alps mountaineering comfort series pads. They are pretty good but are not perfectly flat, they have tube like surface. They are a good option.
Next I bought a bunch of different Thermarest pads. 2", 4", etc. They are really good pads, pack small and are a step above most pads.
But as you tend to see all over... people are downright passionate about Exped Megamat. I am also. They are 4", flat, and pack just as small as any of these others. Oddly though, they have a particular feel to them that can't be matched. The surface/surface tension in the top of them is somehow different and overall much more comfortable. They are pretty close to a home mattress, and I know that sounds like exaggeration.
They have the duo like mine and a smaller one.