My order is 1) light lines, 2) no lines, 3) dark lines. But the fact of the matter is that they're all good, and any pic you draw is better than none >.>

Personally, stick to what you do the best.

I like them all

Honestly, all 3 are done pretty well! By order, really like no line, also really like(but less) dark lines. Light line is the only one that is slightly less good looking...at least for me!

I tend to stick with darker lines as its more reminiscent of traditional art, but light lines and lineless have their charms as well. I'd say pick which YOU like best and see where it takes you ^.^

This may be obvious, but it looks less realistic and more cartoony as the lines get darker. I think it would be interesting trying each method out when you think it fits the overall style or context of a picture, though in the end you should pursue whichever you enjoy the most.

^I second what Ferret said.

I think the lineless is probably best. I'm just happy to see you being more active again though~

Personally, a style like yours just looks kind of ... off, without the lines.

As for the thickness of the lines, it really depends on your mood, and how much detail you want to put into the picture. Thicker lines for a more abstract/iconic look, and smaller for more detailed/realistic work etc...