I would have to say the Ubuntu team is making the greatest contribution to the Linux world. Not because they are bringing advances in code etc, but they have really pushed Linux into the public domain. In the last 3 years, I would say that 90+% of new linux users are using Ubuntu. Why? Because the Ubuntu team has pushed forward and made Linux into a simple, easy to use package for the average person. Making it simple enough to use, but not making it dumbed down that the advanced user feels restricted.
This isn't to say all the other distro's arn't doing so, but Ubuntu is at the forefront, pushing Linux into the public domain like no other. Hell, they got Dell to prepackage Linux. That's a historic event!