Subframe conectors

The "best" brand to buy depends entirely on how you plan on installing them and your (or your shop's) welding and fabrication capabilities.

The US Cartool weld to floor connectors look almost factory when installed. But, they require a TON of welding, some fitting, and are best installed on a car that's been stripped and put on a rotisserie IMHO. Otherwise you're doing a lot of welding, upside down, of dissimilar thickness metals on the bottom of a car. They also require re-routing of brake cables, fuel lines and brake lines depending on how you decide to run them around or through the connectors.

The Mopar Performance connectors are small, I think they're only 1.5"x2". But they're fairly inexpensive, and can be bolted in at home and then welded in at an exhaust shop. They're not the strongest option at all, but if you don't have a welder or fabrication skills they're a good way to stiffen the car up some without breaking the bank. And you can leave the fuel lines, brake lines, and brake cables pretty much in their original spots.

The Hotchkis connectors are beautiful, fit great, and are pretty easy to install. They're also expensive, and still require welding. But probably not any line or cable re-routing.

Or, you can just build your own. If you have some basic fabrication skills and can weld this is a great way to go. 2"x3" tube will not sit flush to the bottom of the frame though, so it will either hang down, or you have to notch the floor in the rear footwell area. 1.5"x3" tube slide over the rear frame rails, sits flush to the bottom of the rails and the floor, and works great. That's what I used on my Duster.