It's more important to pick the right Ph.D. thesis advisor than to pick the right school. Figure out who's doing research that you're interested in, and find out how their students have fared. Once you've chosen the two or three researchers you'd like to work with, contact them and apply to...
I'm interested in understanding foundational issues in QM more deeply. For example, I'd like to be able to read and understand the literature on the Transactional Interpretation of QM and the Many-Worlds Interpretation, as well as the literature on decoherence. What can you recommend for a...