If you want questions to do FOR PATHOLOGY: get Robbins Review and Lippincott's Question Book. PreTest is ok, but should be third.
If you want questions to do FOR STEP WHILE STUDYING FOR PATH: get Kaplan.
If you want questions to do FOR STEP AFTER PATH: get UWorld
Kaplan: detail oriented, requires substantial knowledge fo subjects, many questions, broad coverage. Reference to First Aid and MedEssential pages, decent explanations. Best for use during course work.
UWorld. Concept oriented, hammers home the important topics. Has fewer questions, and keeps drilling the same thing over and over. Great when you put them in the mix for random question taking (as in Board studying), huge gaps in Pathology course work studying. Imensely better explanations and diagrams.
USMLERx consistently falls behind both U World and Kaplan. Most people don't use it. It is the "third player" in the possible qbanks. Use it only after you have depleted both Kaplan and UWorld (aka, you are a crazy gunner trying to do 12,000 questions for Step 1)