Doug Thorley's hands down. They are a tri-Y design, so less bends it seems. If you look at their design compared to the edelbrocks, you'll see it. Plus being that they collect to 3" is a big plus. Not exactly sure the numbers on gain, but they are probably going to be your best gain, since it replaces the crushed y-pipe and restrictive exhaust manifold. I'd say you'll see a big difference in down shifting and acceleration when you stomp it. My best guess is 10-15 hp and torque.
1998 ZJ Laredo, 4.0L, 2WD, it's boring