The best solution for RM /HGEC I have found is Luchaires TGND (which will use HGEC or any exnem based textures - TGND is just meshes plus all stock armour and clothing, you provide the texture of choice, so Exnem/HGEC/F-Inev/Ozmo/EVE .... all good.) - Neck seam gone.
Problem - Though there are quite a few extra Armour/clothing converts in addition to the stock (Shell of the Artifice for instance), since launch it has not been taken up by many other modders to expand it, and also the small wrists of the body leave you incompatible with everything else HGEC (in my case the family wanted The Goddess Store back to so had to switch again) - I think Luchaire kind of gave up after a while, for version 1 of TGND there were a fair few other conversions which have not since been updated for version 2 of TGND, so from the list in her description only about half of those are v2 compatible.
Right now we are using RM v5 with average body and average stock armour/clothing for the same.
EVE for female, and the stock armour clothing, on the desktop we have that with Ozmo's female textures.
And the key I found recently to near perfect seam hiding is http://www.tesnexus.com/downloads/file.php?id=35487
Without TGND its the closest I have come to finding the perfect RM / HGEC no-neck seam or tonal differences setup for all races ( with the exception of beasts which I have a bit of a frankenstein monster of combining Bain installers to sort out
) - Wrye bash helps a lot with these combination conflicts and deducing exactly what textures are in your game.
Edit: Heres a few screenshots in one - this on the laptop with only medium texture setting in game, no anti-aliasing and using hgec/eve with O.F.F, so not brilliant settings to show this but as you can see even with poor graphic settings its pretty good .. http://www.bild.me/bild.php?file=1429328ScreenShot54.jpg - still slightly evident, but on the desktop with ozmo textures completely gone. Thrown an elf male in there too