The GRE physics exam is partitioned into at least 7 different subject areas, so there are no "singly important" formulae, I'm sorry to report. The breakdown is roughly 20% allocated to E&M and Mechanics, and between 5-15% for Quantum, AMO, Relativity, Lab Methods, etc. There's a wiki page with this breakdown, so this may be a good place to look for formulas.
I would say that a lot of this subject matter is encompassed in advanced undergraduate texts, so hitting a good comprehensive text is a good place to start. Further, the GRE is multiple choice, so get good at eliminating answers that don't make sense for dimensional, scale reasons right off the bat, and consider the strategy of just knowing when not to answer since the grading scheme takes into account the total number of questions answered.
There are various study guide and practice tests available at the ETS Website for free, so this would be a good help, and there is an entire forum available for consult on topics related to graduate school and physics at PhysicsGRE.com.