Not sure what you mean by "most effective" but I can share a bit about depth and sensitivity of most of those coils...
For my Explorer XS and II, I've used/owned the stock 8", 10", slim procoil, 13", 15" wot, 10x12 SEF, 15x18 SEF and 18"x24" SEF....just not the 16.5". For both machines and all coils, I could pinpoint like a stock coil. As I walked with the round coils I felt they were easier to swing because my feet bumped into them less than the square/rectangle coils. Also the SEF design has a "V" shape that sometimes catches weeds and overgrown (ARghhh). All coils over 13" felt heavier and quickly fatigued my arm, especially the 18x24. I now use a harness so that helps a lot.
Sometimes the machine went a bit crazy when the coil was reading several targets at once, ie., the screen VDI rapidly flickered between 5-6 target id's. Once I understood that the coil footprint is over multiple targets, it was easy to learn how to pinpoint and dig each of them. BTW, the XS was the hardest, but the faster processor on the II could keep up with several targets. So I imagine the SE would handle the scenario even better than the II.
Didn't notice much difference in depth for the 15" sizes....but the 18x24 was simply impressive. In stable soil, it was 10" deeper than the WOT and 4" deeper than my Sea Hunter 2 (PI) with the 10x14 coil. It was also heavy and so big that it was hard to swing with the stock Explorer setup. To use it effectively, it really needs to be mounted to a long, straight shaft.
I did notice that the larger the coil, the more I had to lower the sensitivity. In some highly mineralized areas, I could actually gain depth by switching back to a smaller coil that worked with a higher sensitivity.
Regardless, all of your choices are excellent. In a lab setting where each coil is equally tested and swung on the same machine, the perhaps offer only marginal performance variations at best. Otherwise opinions are based on a users location, experience and/or lack thereof.
