Loudest and cheapest? Easy, no muffler. Other than that, I doubt anyone here has heard all the options out there, and for sure know one knows what you are looking for. Sound is subjective, hard to type what something sounds like to you. You can get some ideas what loud is, but whether YOU will like it is another story. It's also different on different motors. I had a full Borla system on a '93 4 cyl, header to tail. I swapped in a V6, kept the catback part, and fabbed a new front half. It sounds totally different.
'93 YJ, '02 GM 4.3 Vortec V6, SOA, dual ARB's, 8.8+D30(w/WarnHubs), 4.88 gears, 35" BFG KM2's, AX15, 15x7 wheels, NP231+4:1+SYE+2LO, York OBA, Warn M8000, no top or doors Apr-Oct.
Member and Web guy: New Mexico 4 Wheelers My Truck Page on NM4W Site