The "Loudest" car subwoofer can mean different things, but I'll assume you're just talking about the highest achievable decibel level inside the vehicle.

For starters, there isn't a magic subwoofer that is louder than the rest. It's mainly about the enclosure, and the amplifier. If it's volume you want, go for a ported enclosure. If you want clear, accurate sound, go for a sealed subwoofer enclosure instead. As far as the amp goes, just try to match the numbers. Using components with other components that they're actually designed for can make a big difference.

For ultimate volume, you'll be looking at competition grade subwoofers. Brands to consider would be SMD, Earthquake, MTX, DC Audio, Sundown Audio, and Oroin Audio's HCCA lineup of competition subs.

These speakers are designed for ultimate volume and bass response, and you will need to do some considerable modification to your vehicle I order to run them reliably. This may include the so-called big-3 uupgrade, which involved replacing various electrical mains in your car with 0-gauge welding wire, or replacing your alternator with a high output one. Some enthusiasts have upwards of 3 alternators on a serious setup.

Moving along to the enclosure. You may see an enclosure online, and see that they're quite expensive. That's for a reason. They have been designed by audio engineers to reverberate at a certain frequency, usually around 30hz. I made this mistake in my early days of audiophilia, I considered purchasing an enclosure a non viable option because of the budget, and built my own. Of course, I knew nothing about audio dynamics, and the box came out tuned around to 60hz or so, a bit too high.

This, of course, is strictly speaking along the lines of a plug-in and drop-in system. If you're really going to get serious, fabricating a custom enclosure is the way to go.

In summary, there is no ultimate subwoofer that will automatically be the loudest without any thought. Take your time, do some research and consider the brands I listed earlier. Obviously bigger is better most of the time, but don't be fooled by brands like BOSS Audio that make large subs for cheap. They're trash. Pyle is a relatively terrible brand as well. Remember, if it's to good to be true, it probably is.

Good luck!