Ecology- haven't taken
Microbiology- my course is not relevant at all for the MCAT, it's all environmental microbiology. I like it though, my research is focused in this.
Zoology (invertebrate)- is awesome and I would highly recommend taking an invertebrate course.
Botany- also very cool, very useful in my opinion.
if I were to rank from easiest to most difficult:
microbiology>zoology>botany