I actually had a much harder time getting a science LOR than non-science ones. My humanities/social science classes were small and discussion-based, so it was easy to get to know the professors and make an impression as long as you had intelligent things to contribute every so often. The science classes, on the other hand, were always just "sit and listen to lecture," and I'm not really the kind of person to frequent office hours-- especially if I don't have any questions to ask.
My Asian Religions and First Year Seminar teacher. Not only did he know me really well, but seeing as I spoke openly about my Christianity in my PS, it was a bonus that I could get a teacher who could speak to my tolerance of others' religions.