Always a pleasure to meet a fellow INTJ.
Anyways, Ubuntu is nice for beginners since it doesn't overwhelm you quite as much as some of the more "open" flavors do. I personally have not used Arch yet, nor LFS. However, LFS does seem like it would be extremely useful in learning the ins and outs of a Linux system. And if you're compiling it from source, it should be lightning fast. Which flavor you jump to next though, is entirely up to your tastes and curiosity. Personally, after Ubuntu, I went to Fedora 12, which had a ton of bugs out of the box, but ran like a champ when I worked them out. Some of the other members here have had favorable recommendations for Gentoo and Debian. You could even try this quiz
that is supposed to help you identify the right flavor for you. Exploration is key. If you hear about a flavor that you haven't tried, read about it, and if it sounds interesting, try it out. There really is no right or wrong progression.
Wikipedia has a pretty big list
of various flavors and some of their key differences, that may be worth your time checking out.