Okay, so I just went through this whole hunt myself last month because my kid got into the series at school and wanted to listen in the car. The absolute best version is the one narrated by Fred Berman—it's the original Simon & Schuster Audio production, and he does all eight books. His voice for Michael is spot-on, earnest without being whiny, and he gives Hatch this wonderfully slimy, condescending tone that's perfect. I tried a sample of some other narrator on a UK site and it just felt wrong, like the energy was totally off.
You can get the complete series through Audible, obviously, and that's probably the easiest. But I also found them all on Libby through my local library, which was great because it was free. The audio quality there is identical to the paid versions. One thing to watch: there's an older recording floating around on some free audiobook sites that sounds like it was recorded in a tin can—skip that. The Berman ones are crisp, the sound effects during the 'pulses' are subtle but cool, and you can tell he really grew into the characters over the years. My son and I just finished 'The Prisoner of Cell 25' for the second time, and Berman's performance is half the fun.