Debian is the Daddy of all the derivatives; Ubuntu, Mint, Dreamlinux etc.
Gone are the days where Debian is difficult for new users to install and setup, it also has lots of support and guides on the internet.
Debian Etch (or stable) is secure and robust. The ideal situation if you have your own server, or need speed and reliability.
Debian Lenny (or testing) is ok for those of you who want more up to date packages and have enough knowledge to get round a bit of breakage here and there, but in general, even Debian testing is quite stable. **I must add that at the moment, with talk of Lenny going stable in September, there is quite a lot of movement in the Sid and Lenny repos**
I have Debian Etch as my main stable system for work, with an Xfce desktop, it is fast, secure and reliable. On my laptops and other boxes I also have Lenny and Sid with either Gnome or Xfce, I will always have Debian as my main work distro.
Debian will help you learn more about the inner workings of Linux and other Debian based distros, although not completely the same, Debian knowledge will help you with Ubuntu and Ubuntu-based distros as well.
I highly recommend Debian to everybody, but due to the politics and philosophy of the Debian developers, don't expect too much to work out of the box. Hardware support is getting better, but they will not provide proprietary drivers or codecs out of the box, and their support forums and IRC channels are known to be a bit... er feisty at times
My distros are Debian, Dreamlinux and Mint. Although I do recommend new users download and try anything they can get their hands on, it's fun, you learn a lot, you end up finding what's right for you