Ubuntu is a good bet.
However, I started with debian and went to slackware.
Ubuntu is based on Debian but seems to be more common. However, my computer a dell inspiron 5150 kept freezing. I figured if I was going to do anything important with linux, I better have a more stable distro.
I therefore went with slackware. I heard that it was the most stable or the most basic of the distros. I wanted to learn as much as possible about linux and heard nothing but positive about slackware so I went with that one.