Just pretty straightforward, I am a high school student and like games and I want to develop new distros using LFS(7.3).
So I am confused which host distro to use for my work.Also it should be user friendly and should have a nice GUI.
There's really no best distro, they all have pros and cons. It's like coffee, while there is no such thing as bad coffee, what is the best is a question of taste. When I install Ubuntu on a computer I generally install KDE and LXDE desktops along with the standard Unity and let the end user choose. I consider Ubuntu and variants to be very user friendly until you get into the really advanced stuff or try to install an incompatible hardware/software and have to use command line but then if you're planning a LFS it's good you learn to use the command line anyway.
+1 to Ubuntu/Kubuntu etc. Both of those and Xubuntu are excellent choices, probably Lubuntu also though I've not tried it. If it's an old and/or cheap system, go with Xubuntu or Lubuntu. Else it's just down to your personal preference. Less customisation options in Ubuntu than in the other three though.
I prefer to use CentOS, mostly because I use RHEL at work, so at home I write scripts to test out on my CentOS VM before I migrate them over to my machines at work.
If you want to get some decent experience start with no GUI at all (Ubuntu Server is a good starting point for this), then attempt to download your GUI of choice. Try to compile it from source, install it, and configure it. This will give you decent insight as to how the X-Window system works, what can go wrong (alot can go wrong) and how to fix it. Most of the problems I see at work with Linux involve X and problems with the GPU.
As others have said there really is no best distro, just find what suits your needs and tailor it even further until you find a good fit. Ubuntu is a great choice because of the large community out there, just experiment and have fun.