Best product? Your brain and your eyes. And they're free too.
Like others have said, it's more important to learn how to use the detector than trying to determine which one is the best.
Personally, I use the Valentine 1. I generally only use it when traveling up and down the 5 freeway going from SoCal to the Bay Area and back. Very useful (the arrows and bogey counter) since CHP likes to travel in packs now. It used to be pairs a couple of years ago, but this memorial day weekend, I saw them in groups of 3. They'll have the lead car buzz by at like 90-100+, which scares everyone but then lulls them into thinking the cop's gone. Then, cop #2 will be an equal distance behind, usually right over the horizon, and zoom up to catch people as they speed back up. Now they've added cop #3 to get people cop #2 missed.
You have to remember that radar detectors don't detect when cops are around, just when they use radar. On the 5 and the Bay Area, the cops tend to cruise with radar on so it's very nice. But, if they use an airplane (the main reason why I have a moonroof), or just go by pacing, then you'll have to constantly scan (which you're supposed to do anyways).