Hasn't been mentioned and it'll really depend on how keen you are to learn, but if you start out on Slackware (or a heavily Slackware based distro ie. Vectorlinux), transition to any other Unix/Unix-like OS will be easy because it's very clean and not a mess of distro specific scripts and config files. The flipside is the learning curve is steeper, but you will learn and you wont be playing linux, you'll actually know what you're doing.
On the other hand if you don't want to dive into the deep end then I'd suggest going with Ubuntu, it's pretty painless to setup and get everything working in most cases and has plenty of communities around the web, so you'll be able to find help easy, theres lots of guides, how-to's etc. Any problem you have with it, someone will have likely been there before and google will find you a solution.
On the other hand if you don't want to dive into the deep end then I'd suggest going with Ubuntu, it's pretty painless to setup and get everything working in most cases and has plenty of communities around the web, so you'll be able to find help easy, theres lots of guides, how-to's etc. Any problem you have with it, someone will have likely been there before and google will find you a solution.
