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.
You probably want to decide what kind of sound you're looking at as well. A loud exaust that lets you easily distinguish the RPMs by tone, that u might stereotypically find on the end of a crx? or more of the 3" flowmasters on my old firebird 5.0. You can hear both from around the block, but they're entirely different sounds. Just my Thoughts. I'd give you my opinion, but I don't know much of anything about mufflers / exaust systems on jeeps. Good Luck. -Matt