I think ocr are best for sciences. Aqa marked 3 out of the 4 papers I sat with them more than a grade out this year. I think generally ocr give a fair paper from the experience I have with them
I chose to do AQA for sciences as the syllabuses looked most straight-forward. I was proved right; they were nice courses & concept based which made them easier to learn & the papers more straight-forward. The practical exams were easier to get high marks in as well than the coursework for other boards (at least I thought so).
OCR chemistry & physics are context-based which is interesting, but makes studying the actual underpinning scientific facts more time-consuming and the papers more complicated. OCR Salters in particular is a nightmare. (I tried to start OCR and switched to AQA).
I have no knowledge of the Edexcel science syllabuses, so I can't comment.