Many years back i was learning slackware , which i think along with Gentoo is the best for getting a real understanding of linux.These skills can then be easily transfered over to any other distros.I started by just reading the offical slackware pdf and not using any gui , only command line.This in itself was the biggest help , as i could easily switch over to another distro and be comfortable using it.Not sure what its like now but i think by starting with ubuntu etc is just wasting time and effort.