CreativeNerds has a pretty good list of GIMP tutorials, and this blog has some nice Inkscape tutorials. But I don't know if they are "the best, easiest-to-follow, most comprehensive series of online tutorials available". Why don't you post your question to the community here, maybe other users prefer other tutorial websites?

Also, you can find a larger number of tutorial websites & blogs via delicious.com/gimpgate/tutorials, but most of them are for GIMP, not Inkscape. I haven't really had the time yet to add more Inkscape-related resources to the link collection, but it's on my to-do list. If you happen to find a good resource, please feel free to share it with us :)