Which Linux (Mandrake, RedHat...etc) is the most difficult to use?

Making Linux from scratch is the easiest. After that, it's all opinion. I'd say Gentoo is hardest to install, but Slackware is harder to use after that.

But are they have a large differences in term of its usage and functions?

Nothing is as hard as someone makes it out to be. Some find Slackware the easiest as it uses simple startup scripts and configuration files you can simply edit manually yourself.

Other find Debian the hardest cause compared to other distro's, it has the hardest install, besides Gentoo for others and LFS which I don't really consider a main type distro for the average user to get a system up and running; more along the lines as a learning distribution to see how Linux works, what's needed, etc.

Others will tell you Redhat or Mandrake are the easiest cause they have nice GUI type tools to edit or change configurations. But if you did a comparison on a Redhat system's startup scripts compared to Slackware, you'd find that if you were to edit by hand these files, Slackware would be easier to understand really.

So its all really your preference in thinking what is harder and only how you make it out to be...

Regards.