1. What is the best distro you have used?
FreeBSD
2. What is the worst distro you have used?
Slackware
3. What is the most challenging distro to learn?
After you learn one Unix/Linux system, the rest are easy
4. What is the most user friendly distro you have used?
XanderOS or Ubuntu (Choose your flavor)
5. Do you use your distro for everyday use? Why?
Yes, they're on my primary desktop, and servers.
The biggest question that you need to answer for yourself, is how much are you going to be working from a command-line environment, and how much of a GUI do you need in a unix/linux environment.
The more GDE and GUI elements that you install, the more power and drive space your distro is going to command of you. And, just as important is WHAT graphical desktop environment you choose. Gnome, KDE, or a Piece Meal desktop. Personally, I use XFCE4, and it's pretty inclusive, but does take a command-line knowledge of your system, which some people aren't/can't invest the time for.