I skipped a lot of opening acts this year, much more so than in past years, so I'm not sure I'm a good reference, but Modest Mouse opening for R.E.M. at Merriweather is probably my least favorite opening act of the year. On the other hand, The National who played before the Modest Mouse at the same show were easily the best. I've seen The National play several times at the Black Cat and the 9:30 Club, and their performance at Merriweather was the best I've seen them. Sadly, most of the audience hadn't shown up yet.
My least favorite headliner this year was Margot & the Nuclear So and So's (2008/08/13) at the Club. I'd seen them before at the Black Cat, and I thought they were very good, but this show was just lousy. They played mostly new material, and their new material just didn't seem at all interesting.