Ask around at your school. Cell bio at my school is probably the hardest to ace. The material isn't difficult, but it's really detailed. Our exams were short answer, and you had to answer in the space allowed. The professor would specify how many sentences you were allowed, and if you wrote more, he'd often give you zero points. You could get a B or better by studying the slides he really focused on, but to get an A, you really needed to have assimilated some really random details from the suggested reading. I remember that on one exam, there were three questions based upon a single sentence from the text that was pretty obscure and not covered in lecture.
I didn't take genetics, but that professor is also reputed to make exams extremely difficult. Biochem isn't an easy A, but it's easier than the other two in that you can figure out from lectures what they'll put on exams. Biochem I & II are both easier A's here than cell bio and genetics.
I'd ask for a lot of opinions and choose based on which professor is known to be the most straightforward in testing, and factor in how interesting their lectures are, unless you're the type to skip lecture and focus on the posted material.