Once A Metro, a New York Red Bulls fan community, is currently polling their reader base to determine which MLS teams they despise the most. The Philadelphia Union, probably mostly due to pity, currently sit just fourth in total vitriol. More competent rivals D.C. United, New England Revolution and New York City FC currently rank first, second and third. The most hated Western Conference team is obviously the LA Galaxy, currently the 6th most hated club.
It would certainly be interesting to see which rival club is most hated by Union fans.