My experiences haven't been with a very wide variety of airliners, but I've spotted a lot and stood at the end of the Hopkins Airport (CLE) runways 23R/L... So here's my opinion...
Loudest I've ridden -- DC-9-10. I was sitting right next to the engine on the left side and it took me 10 minutes to get used to the noise. I couldn't hear the flight attendant doing drink service so I used hand signals to order. Luckily she understood me. The Fokker 100 comes in close second, but the pitch is higher and not so much of a roar as a loud hum. (As a musician I'm sensitive to that aspect).
The loudest I've heard taking off (from about 50 yds. away) are DC-9, Boeing 727, MD-80, and one awfully loud Learjet (private) whose pilot/owner is going to get fined someday for violating Stage III compliance. Boeing 737-200 can be quite loud too, with those old "tube" engines.
Personally, I've seen a correlation between the operating airline and the noise the jets make as well. For example, Continental's MD-80s are loud, but not nearly as loud as Delta's. Northwest's DC-9s are screamers, but I've been deafened by Midwest Express. USAirways Metrojet and Southwest Airlines both operate Boeing 737-200s from Cleveland, and those are loudest among 737s.
In terms of loudness when riding on the planes, I think it really matters the location of the engines and where you are sitting. There's going to be a lot more noise if you're in a DC-9, MD-80, F-100, EMB-120, etc. with fuselage mounted jets than if you're on a plane with wing-mounted jets, just by dint of distance. Also, if you're sitting right over, next to, or below a jet, you're in for an earful.
I also nominate the DeHavilland Dash-8 in the turboprop category. Not only is it loud, the props vibrate the entire aircraft which was a living hell for me when I was riding with an ankle injury!
Quietest? Hmm... Most of the turboprops are pretty quiet in comparison to the jets! I would nominate the Boeing 757 and the Boeing 737-800.