AFAIK, this is the list of the movies. Unless I'm missing something.
X-men (2000)
X2: X men united (2003)
X-men 3: The Last Stand (2007)
X-men Origins: Wolverine (2009)
X-men: first class (2011)
The Wolverine (2013)
I have seen the first one in 2000 and only bits and pieces of one or two of the others. With the exception of The Wolverine, which I recently watched.
There is mixed suggestions for the best order to watch. Some reports say watch them in the order of release but I like the story order. I think... as long as it doesn't spoil anything to watch it that way.
The it would be something like...
X-men: first class (2011)
X-men Origins: Wolverine (2009)
X-men (2000)
X2: X men united (2003)
X-men 3: The Last Stand (2007)
The Wolverine (2013)
... or would it?