Harry Potter and the Order of the Phoenix, and here's why:
Out of all of my Blu-rays, this is the one with the best transfer. It's ridiculous how fucking good the transfer for this film looks. I watched it again last night and shit, it's pretty damn incredible.
Pan's Labyrinth has about the same quality of transfer to it, if not better. Plus, seeing as you already have Babel, you might be interested in the Charlie Rose episode (sadly, standard def) bonus feature which has an interview with Del Toro, Inarritu, and Cuaron.
Close Encounters is about as nice of a set as I hear the Blade Runner set is. The transfer's decent, you get all three versions of the film on one disc, the other disc is loaded with HD special features, and it comes with a comparison chart. Plus, the box is damn pretty.
Other than that, use your own judgment and movie tastes, there's a pretty good selection of films on Blu-ray all ready, and even the shittier transfers look better than DVDs.