Witch one is the best is quite hard to say as it comes down to personal taste.
If your new to linux I would recommend to try a few of the LiveCD options on the computer before installing. Personally I run Puppy linux
on my Acer Aspire 3000. It is small (100++MB). Works on most hardware I have tried it on (I have it running on a 400Mhz, 64MB laptop. And provides most of the basic applications. Its not for you if you want to try out the latest of every application . A lot of people like Ubuntu
as it is quite polished. As it is use debian
as it's platform.
If you want to learn the internals of Linux you should take the time to study Linux from scratch
. Preferably in a virtual environment like VirtualBox
. The later runs and installs nicely in the latest Ubuntu release.