Ubuntu is the most popular for desktops, Red Hat and Suse are geared towards application web servers,AIX (IBM) is geared to the Enterprise SAP servers and the like. Solaris (Sun) is geared to your big databases like Oracle.
Ubuntu by far ... Very easy to install (easier than Suse), and great support for most hardware (better than Redhat) - I am running ubuntu on my HP notebook, and everything from my MS mouse to my Wireless network card is working, with very little effort.
I once installed and tried to use lunix, it didn't want to see my CD-ROM drive. Apparently I had to mount it and that just sounded too perverse, so I went back to Windows...
I installed ubuntu a couple of months ago and had minimal problems in getting things to work (only my Toshiba printer/copier is unsuported) I searched the ubuntuforms and found answers as I needed.
I find linux akin to using DOS with windows with about 20 commands you need to know, others you can find as you need.
Kubuntu for ease - especially if u still want a familiar Windows look and feel to start with, but Slackware if you really want to learn - Slackware really makes you work, but you find that u learn a lot more