A:Far and away, the self-inflating kind. Not so much due to comfort, but because the old-style "air mattress" you're thinking of are co-o-ld. Self-inflating pads have insulation. This is a key factor in almost all conditions, as ground is cold almost everywhere.
The other big factor is weight. In my mind you want about the lightest pad you can find. I really like REI's Lite-Core 1.5 ($75). It's pretty comfy, and weighs only 27 ounces in the full-length version. The Cascade Designs' Therm-a-Rest ProLite Plus ($100) also is an excellent lightweight pad.
Now, as for comfort, probably the best out there is the Exped Downmat 9 ($199 in full-length). That's because it's an air mattress with a down filling. The down offers insulation and some cushioning. Very plush. But...weight is around three pounds.
A good alternative is to combine a very light self-inflating pad (even a ¾-length version) with a light foam pad, such as the Cascade Designs Z-Lite ($40). It's a very light (14 ounces in full-length; 10 in ¾) foam pad that really gives a single pad a comfort boost.