Now, while some people will swear up and down that Slackware or Fedora or even Puppy, for that matter, is the best Linux desktop, I think the answer is more complicated. In fact, I dont think there is a single answer.
I think the best Linux desktop is the one thats best for a particular person based on their needs and level of Linux expertise.
So, the next time someone asks you that question, I suggest you reply with a couple of questions of your own.
For example, you could ask, "Do you want to replace Windows? For home? For work? Are you interested in Linux because you want to get some new life out of an old system? Do you just want to mess around with Linux?" And so on...
Then, once you know where theyre coming from, you can give the best possible answer.