First of all, never go with a cheap pad to save money. The pad can make the world of difference in the life of even a cheaper carpet. As everyone knows the traffic a carpet will receives will help make some of your decisions. I prefer a higher density product (7lbs or more), but don't get a rock hard pad. If you can afford a pad in the 8 to 11lb range, Check it out the carpet you have selected or various carpets to get a feel. **Remember to try the combination over your substrate (i.e., plywood, wood or concrete) notice the differences. Walk on the combo, with and without shoes. If you have a good flooring contractor, or retailer, they should be happy to supply you with the test materials. Remind them a happy customers will usually recommend them to their friends.
Another thing you should know are the types of pads (i.e., rebound and slab/monolithic). How do tell the difference?? A rebound, looks like a bunch of multi-colored, chopped-up pieces of foam that has been fused together (look for one with a webbing mesh, sign of better quality). Remember your density numbers!! A slab/monolithic is just as it sounds, one continous piece and color (Stay away from the thin spongee green contractor grade. It dead pans in high traffic areas and under furniture). Your higher density is critical in these pads. I believe "SCI" still make a high density pad (dark gray in color.) It more on the commerical side, but will worth it if don't plan on moving anytime soon and can afford the extra cost.
Remember, also pad thickness is important. Higher density, you may can use a thinner pad and achieve the results you require. Thicker is not better!!!
I recently, used a 8/9lb range rebound, under a berber for bedrooms and great room area. The neighbors and friends when nuts and all wanted to change out their carpets.
But remember, everyone is different and that why I suggest the in home test for a couple of days or a week. Don't rush into a decision.
Commerical and residential pad use is quite different. I am assuming you are looking for residential?
As for wet areas/basements, that one has every flooring person puzzled. The first step is to do a moisture test on your concrete slab (I'm assuming concrete slab.) You can get a kit from a flooring contractor, have some one do it for you or go the cheaper route. The cheaper route, is the easy. Cut about four or five piece of plastic (i.e., a ziploc bag, plastic wrap or 8mil polyproylene - ziplocs are quicker to get out of the cabinet). Chose the four or five location on the basement slab to test (i.e., middle of the floor, near entries, near outside walls, etc.). Tape the plastic down on the concrete, creating a seal. Watch for moisture to collect (up to a week). If no moisture, you have it made, go with whatever your heart desires. You are also lucky, this doesn't happen very often. When you do the test, try to plan it during a high moisture time (rain in the forcast??).
If you find moisture, contact a floor contractor that knows the business. They can give you ideas, such as a concrete sealer??????? or constructing another substrate such as water grade plywood prior to carpeting the area.
Best bet is to test the area and then go to someone you trust and ask for technical literature for below grade/ground and wet areas. Also, search the net under some to the major flooring manufacturers, such as Armstrong for recommendations.