Back in 91 when I finished by 81 Z with my dad in time for the beginning of my jr year ... I thought I was going to go deaf. The road noise was simply maddening. I bought a "sound dampening" molded carpet set and if it worked, I can only imagine what it would have sounded like without it.
I bought my stereo, speakers, subs, and amps at a car stereo shop in a larger town about 30 minutes from our house ... and they were pushing "dynamat" at the time and I passed it off as useless or frivolous ... until that first round on the road.
I parked the Z in the shed, removed my interior (much to pops' chagrin) and saved two paychecks ... and bought enough dynamat to do the entire interior. Headliner, doors, and floors. The difference it made was phenomenal. That was over 20 years ago.
I did this sort of work through highschool and into college ... and obviously it is easier if you're dealing with a fresh build. The products have gotten better since then ... and it depends on what you pay for.
Don't get asphalt based materials. I've had guys tell me horror stories about the smell on hot days. Most of what is out there is pure junk. A single hot day leaves a lot of sagging headliners.
Second skin is by far the best. After that I would say Dynamat extreme products. If you can find the original and just need to do your floor ... this would be the most cost effective. Another product is Fat Mat ... but only the Mega Mat line is worth a damn.