The one you’re running.
A bit of a simplistic answer but there’s a great deal of truth to it. It is too easy to get distracted by the new shiny and keep changing distributions. When the time could be far better spent just learning Linux – to a great extent all Linux distributions are the same. You can get Firefox (or whatever browser you prefer) with any of them; similarly LibreOffice is nearly always available. It’s the software you use on a daily basis that is important; not which distribution you’re using.
Similarly the desktop environment you use is selectable – this laptop has a distribution-specific flavour of GNOME, Awesome, Xmonad, and i3 (although I spend most of my time in Awesome). You might be able to tell something about my preferences for “desktop environments” from that list! A whole new desktop environment and a whole new look is just a quick software install away.
And a whole lot quicker and less disruptively than you can install a different distribution.
Different distributions offer different feature sets and different system administration commands (dpkg vs yum), but it isn’t that difficult to adjust to these differences especially when most of the time you are just using the computer to do real stuff rather than just managing it.
