Boxing
Jiu-jitsu
Judo
Wrestling
Muay Thai
Something else
Which art is the toughest to master and why?
I'm going to say Judo.
Possibly Boxing as 2nd.. although i'm not sure as to how anyone would "master" boxing...
That is difficult question because there are so many subtleties in each style.
Boxing is extremely tough to master because you are so limited in your attacks. You have to develop insane timing and creativity to outwork a good opponent so I would almost vote for it on that merit alone.
Not saying it is the most effective style or anything, just that you don't have many ways to win so you must be very, very good with your tools.
Chuckles nailed it.
You could say "well, muay thai entirely encompases boxing, so it must be harder" but boxers are MUCH better with their hands than Muay Thai fighters, so getting as good with your hands as a muay thai who had "mastered" muay thai, would not be considered mastery in boxing.
TTT
I know it might seem to be the easy way out of the question but what would you define being a master as? The best of the disciplines constantly admit that they are always learning and get better every time they practice their art. What constitutes a master wrestler? a NCAA title? Does a master in boxing need a belt?
Trick question. The correct answer is all of them. You can dedicate a life time to each and yet know nothing. This is why they are called an art. Art is viewed differently from every angle. 
OneGloveJimmerson - I know it might seem to be the easy way out of the question but what would you define being a master as? The best of the disciplines constantly admit that they are always learning and get better every time they practice their art. What constitutes a master wrestler? a NCAA title? Does a master in boxing need a belt?
For this thread I will define "master" as someone who has the ability to be competitive at the highest level in his arts sport.
croy_00 - Trick question. The correct answer is all of them. You can dedicate a life time to each and yet know nothing. This is why they are called an art. Art is viewed differently from every angle.
This 
[quote]MentaL - Etch a sketch [/quote]
Hardly. I did this after just one week of Etch-A-Sketch training:
BJJ
Actually - I've read some studies that showed any physical endeavor practiced with the intent to improve is mastered to your level after 10 years. So, you'll be as good as your going to get after that decade. So, to answer the question - they're all the same and mastered in the same amount of time.