2008 because the past 5 years has mainly been the same side and it has been the best period of United's history. For the first time they have truly found the right balance for sustained success at home and in Europe. 4 titles in 5 years and 3 Champions League finals is a great record by anybodys standards.
I went with the 85 side, my first games i attended were in that season, brings back good memories, although they dont compare in terms of quality to the others listed.
I went 2008 as well. Not just because it's the modern generation, but that team for the most part (apart from Hargo, Nani and Ando) were part of the revolution after our barren spell in all competitions where we watched Arsenal's invincibles and Mourinho's (and Abramovich's) Chelsea more or less thrash us for 3 years in a row. Not to mention failing in Europe miserably in 2005/2006. We had RVN leaving and Roy Keane in that season as well and the turnaround that we started in the summer of 2006 was astonishing considering certain journalists believed that United's time at the top of English and European football was over. And as someone else has mentioned, the consistancy of this team at home and in Europe has been better than any other previous team, considering that we are playing along with one of the greatest teams in history in Barcelona.