What are the most common UVs? The Genesis default UV is V4, and the coordinating textures for Four Arms fG and Triclops are M4. What else? Since I use Genesis almost exclusively now and DAZStudio usually switches the UV to match the texture automagically, I haven’t been paying attention to what UVs I’ve been using.
I really don’t think you need to worry about much besides M4/V4 at this point. Gen3 textures need to be converted before they can work with Genesis in any event, so whatever you do for M4/V4 would work for the up-converted textures. I don’t think F4/H4/A4/S4 had their own UVs, did they? I’ve got all of those figures, and none of them seem to have a listing in the Surfaces tab. I know they did in Gen3, and then DAZ seemed to get rid of most of that for Gen4, and then, realizing that they were leaving money on the table, brought it back for Gen5. Really wish they hadn’t, but such is life.
Gen5 has very very very few commercially available merchant resource kits at present—I haven’t paid attention to what’s out there for V5, but there’s precisely ONE commercially available M5 merchant resource kit that I know of, and I really don’t think people are going to be trying to darken it—so I think you’re OK to ignore Gen5 for now.
And if you try to get into supporting the option for M5/V5/A5/H5/F5/S5/YT5/whatever5 is next, you’ll make yourself crazy.