If you doesn't got a computer with much RAM i would way Slackware or Gentoo.
Why?
Becous they are stable and doesn't require a computer with much RAM.
Gentoo may be hard to install, but if you fallow the great, i realy mean great guide on their site it shouldn't be a problem. And, you'll learn much of it too!
Using portade may be time consuming, couse it compile everything but things does get better perfomance that way.
Portage does install everything by itself, even if it does compile everything, it does it automaticly.
Slackware is stable, fast and isn't to hard too install either. On the other hand, you have to compile most of the things by your self and there's nothing that checks if you got all the packages that a program needs. On the other hand, trust me, you'll learn a lot more things by using Slackware!
In the end...go for Slackware if you want to learn stuff about the system itself, go fot Gentoo if you want to learn how to install a system...manually...and want a good installer tool (portage is great, but it's slow (becouse it compiles everything automaticly!).
If you want enything else, go for Ubuntu. It's nearly perfect (first Gentoo, then Ubuntu and third Slackware (becouse it isn't so easy to install stuff!)!
