Which mod should I use in conjunction with HGEC to remove seams from my character?

Luchaire's Body Seam Remover / Neck Seam Reducer are both good, but you will not be able to completely remove the neck seams, as the chargen changes facial skin tone far more than the body skin tone. Optimized Facegen Files helps with the default skin settings, but all bets are out the window once skin tone is changed in any way.

Someone did a set of "choker" amulets, so you can hide the seam under jewelry. Don't remember who, or the mod name, sorry.

http://www.tesnexus.com/downloads/file.php?id=18669

or

http://www.tesnexus.com/downloads/file.php?id=34082

Also bear in mind that most things used to correct this try to hide the problem (like the concealers above). It's pretty much impossible to solve the issue due to the way the head, feet, hands and body are connected.

I'd say it has more to do with the skin tone tint adjustments not matching than the way they connect. Even without IFT, OFF, LBSR, etc, the default skin settings don't look that noticeable. It's really too bad that there hasn't been an OBSE plugin that allows face and non-face tints to be modified at the same rate...

For Luchaires HGEC Body Seam Reducer there is the side effect that High Elves will have no body texture and yellow heads. The workaround for this is http://theelderscrolls.info/?go=dlfile&fileid=346 by bg2408. You will need Wrye Bash to add tags. The author did not specify which tags, but I assume they are these:

R. Ears
R. Head
NoMerge ( I'm not sure about this one, but Wrye Bash will likely merge if this is not added. I think the changes should be imported, not merged )

Still, this fix ( for me ) makes High Elf heads still a bit yellowish. I had to go into Photoshop and adjust the head and ear textures by hand and with my eyes.