For me Immunology was one of the coolest classes I took as an undergrad. The immune system is so intricate that it makes your head explode just thinking about how it all evolved. I would definitely take it if I were you. Plus, it will help you tons in other upper level bio courses like microbiology/virology/biochemistry.
As far as the MCAT is concerned, I would say take genetics. I had at least 1 passage dedicated to genetics, and several independent questions asking me about genetics questions. Plus (well at least when I took the class), the questions on genetics exams really made you think critically, kinda like the MCAT does.