Well this has been a source of controversy, some say it was Tim Miller or or someone close to him at Blur Studios, but it may actually have been Ryan Reynolds himself:
We May Finally Know Who Leaked The Deadpool Footage
While Miller hasn’t publicly said he was behind the leak, numerous
sources point to him or one of his colleagues at Blur. I have no
smoking-gun proof other than my typically very reliable sources on
such matters. But if so, it was a bold move from a filmmaker who had
nothing to lose and everything to gain.
Ryan Reynolds has stated in an interview on The Tonight Show with Jimmy Fallon that is was it was one of 4 people, him, Rhett reese, Paul Wernick or Tim Miller, but it might have actually been the actor himself who leaked it:
We developed the script 6 years ago, wrote this fantastic script , it
leaked online and Deadpool fans went nuts for it. So the studio
granted us a small amount of money to make a test footage that we
shot, which then sat on the shelf for 4 years as it does and they did
not do anything with it, as it does and just under 2 years ago it
leaked accidentally onto the internet... Well here is the thing, the
deadpool fans freaked out and overwhelmed Fox and Fox basically had to
green light the movie, the problem was that the footage was owned by
Fox and sort of illegal that....I know that one of us did it, there
me (Ryan Reynolds) Rhett reese, Paul Wernick, Tim Miller the director,
one of us did it. We said that someone should leak it.