have to say the 8x9 or 10x12. I will add that there are times when <EM>"better depth"</EM> is obtainable by using a smaller coil, such as a 7" concentric. It really comes from trial and error, using two or three sizes in your hunting environments on your detector.
<BIG>►</BIG> 7" CONCENTRIC DEPTH ADVANTAGE OVER LARGER COILS: I have found that when a site is trashy, coils larger than 7" can be hamperd by masking problems while the 7" can "work between" better and thus get "better depth." Additionally, I have worked a few sites where the mineralization effects cut the performance of larger coils, thus allowing the 7" to actually get a little "better depth."
<BIG>►</BIG> 8x9 CONCENTRIC DEPTH ADVANTAGE OVER SMALLER & STOCK COILS: I personally do not care for the brown colored coil, but that's a personal thing. Often you can't tell much difference between close-sized coils in many applications, but after spending more time afield I have found the 8x9 'spoked' concentric coil to give just a tad bit better depth. No, the difference isn't like night ☻ and day ☺ but I have noted a little improvement in depth on smaller-sized coins.
<BIG>►</BIG> 10X12 CONCENTRIC DEPTH ADVANTAGE OVER ALL SMALLER COILS: I AM a big fan of smaller coils in the 4"-7" range, and have never big a big fan of larger-then-stock coils. That's not to say they don't work, just that I don't have that many applications for them. Still, I reflect back a number of years (15+) to when I did have some use for a larger coil on plowed fields and football fields. I opted for the 10