Schools who would accept someone in the bottom 50% of physics GRE scores but have extensive undergrad research experience.What do you mean by "second tier" and "best"?
Preferably the U.S.Are you looking at anywhere specifically in the world?
I would like an even blend of theory and experiment/observationYou then have the question of what areas of Cosmology? Are you interested in experimental (e.g. Planck), theoretical or data analysis (e.g. parameter estimation)? Are you interested in simulating large scale structure? General relativity? Braneworld cosmologies? CMB? Astroparticles?
It may be helpful to list a handful of some of the areas of interest as this can help narrow down the list of institutions pretty quick. For example a university may have a very good cosmology program but if they're purely experimental then it wouldn't be suitable for someone interested in something like string cosmology etc.
Charles Bennett, the PI of WMAP, is at JHU. Riess was at Berkeley when he discovered dark energy: http://adsabs.harvard.edu/abs/1998AJ....116.1009RJHU's Dr. Adam Riess is the one who lead the team that discovered dark energy, for example. It's worth looking in to.