There are still variables you must list, to get an answer to your question. How mineralized are your beaches? You can tell the amount of mineral by the color of the sand. The darker the sand, the more mineralized it is. So for example, white sand would be the most-mineral free. Cinimon tan color would be moderate. Greyish dark would be getting tougher. Metalic grey/black (like what you'd see in gully washes) is really bad, etc.... So you must be specific on the type beach you're going to be working.
And are talking "the best" for depth, with no other qualifications? Because the knee-jerk reaction for that is: any of various beach pulse machines. I mean, heck, you could even get some of the pulse nugget machines, and get a nickel or gold ring to 1.5 ft. down! I've seen some guys do that, and believe me, they don't miss a thing, and double the depth of other guys out there! But naturally with pulse machines (especially a nugget machine), you will hear every single paper clip, nail, birdshot, staple, etc.... If you are on a beach riddled with nails and iron, you might end up hating life, and wishing you had a standard discriminator coin/jewelry machine, like an Excaliber, CZ6, XLT, or whatever. But of course the downsides of those are, they will not cut through the nastiest minerals (but do ok for all other wet salt beaches), will not go as deep, and will not find dainty little tinsel thin gold chains, etc....
For example: where I'm at, the minerals aren't un-bearable, so having a pulse, for purposes of punching through black-sand, is not an issue. We also have beaches that allow beach bonfires (which introduce nails via burned pallets) and various industrial history (wharves that have burned down, beach side industry, old dumps in the sand dunes that are now spilling out on to the beach after storms, etc...). So for those of us who strictly hunt after beach storm erosion, where depth is not an issue (mother nature puts everything into pockets/zones where speed is the name of the game), digging 20 nails to each conductive target can be a real downer. I've seen people try to use pulse machines after storm/swell erosion on our junky nail-riddled beaches, but they usually don't last long. Perhaps on touristy clean beaches, or beaches where nails are more spread out (allowing you to guess tones on a case-by-case basis) they would be more at-home?
So it all just depends on your goals, the trade-offs you are willing to live with, etc.... I use either the explorer or Excaliber, depending on if I'm going to be getting wet or not.