UsmleRX during school, then Uworld.
RX's explanations are a bit nauseating sometimes with their comma delimited lists of symptoms. For example, "Neurofibromatosis type 1 presents with cafe-au-lait spots, iris harmatomas, pheochromocytoma,..."
But I would take their explanations over Kaplan anyday. Kaplan's explanations are too brief. RX reviews the associated disease with each answer choice and presents the relevant page in First Aid. If you read the entire FA page after each question and you will have read FA many times over.