what makes one better than another? Depth is the usual standard of our wishes. But even within that arena, is loads of "buts" and "if's" and "although's". I mean, there are detectors that can detect a quarter to a good 16" or more with ease!! But they will be practically useless in all but the cleanest white dry sand environments, because they're just too plain squirrely (squeel off over every little grommet or nail, with no tell-tale way to tell the difference). Or perhaps they get this deep, with reasonable ID's (reject iron, etc...) yet they falter when they get over mineralized ground. Or perhaps they get deep AND work well in moderate minerals, AND retain fairly reliable TID down to their deeper reaches, however: maybe they mask with junk targets nearby? (the common compromise of the power-house machines).
So you can see, that to answer "depth", as a factor is "what is the best detector", can itself, be a loaded answer. Perhaps you'd take this supposed depth demon power house machine to a nail/junk riddled ghost town, or set of ruins, or urban demolition sidewalk tearout, and get your sorry little *ss kicked by a 2 knob little 2-filter shallow seeking wimp machine, simply because he can effortlessly see through and around small iron? Which machine is "better" in that case? The machine that went deeper, or the machine that wasn't as deep-seeking?
So your question is sort of like asking: What is the best car, and what makes a car the "best"? A person would just turn around and ask you: "do you want speed? Consider a corvette". "Do you want low end torque to pull out tree stumps? Consider a low geared truck". "Do you want to carry kids to soccer practice? Consider a mini-van". "Do you want great gas mileage? consider a honda accord", and so forth. Your question is too nebulous.
