Four factors are key. Cost, comfort, weight, and adequate R-value for the conditions. All three are critical in my estimation.

Comfort: Generally the thicker the better. I find the NeoAir to be wonderful. The Thermarest Prolite is ok and the Prolite Plus quite nice. I hear good things about the Big Agnes pads, but have not used them.

Weight: The Neoair is tops here, as a bonus it also packs the size of a water bottle.

R-value: The NeoAir, Prolite (regular), and the uninsulated Big Agnes pads might not be adequate in very cold weather, but are probably OK for three season use.

Cost: The NeoAir is my top pick for most conditions and is even in the running for cold conditions if paired with a Thermarest Z-lite, but it is very expensive. The regular Prolite and Prolite Plus are a good bit cheaper. There are cheaper self inflating pads similar to the Prolite and the Plus that are cheaper, but I have no experience with them.

Some add bulk as a factor, but I don't see it as critical since you can strap even the largest pad on top of the rear rack. That said the NeoAir and Big Agnes pads pack very small. I see it as a nice plus, but not critical that they can ride in a pannier and not take up much space.