Which is the best order to teach IB Biology?

Every school will have unique circumstances that will dictate the order of the topics in Biology. The combination of HL and SL classes, climate, school location, national programs, teacher preferences will all factor in on the choice of order of topics.
Below are four suggested teaching schemes for different situations.

Teaching SL and HL students in the same class

This plan assumes that some lessons each week have both SL and HL students and there is probably one lesson each week with only HL students. It is sometimes possible in a school to be more flexible with the lessons if the SL students have a 'study lesson' while the HL students have the single lesson. It is advantageous to put two or three HL lessons together and to complete a chunk of a topic in one three lesson sequence, then teach the SL students with HL students for all the lessons in the following two weeks. This makes for better teaching and learning of the HL topics. I think it's important to give the SL students plenty of notice about the HL lessons so they can prepare to get other things done during this week.

A mixed HL and SL class in a two semester school year.

Teaching SL and HL students in the same class with two teachers

This model assumes that there are two teachers sharing the class with half of the lessons each.  There are advantages because by the time teacher A reaches the cell membrane section of topic 1 the students will have learned about phosphate, lipid and the 3D structure of proteins.

Thoughts about teaching mixed classes

I have known a biology teacher teach only the SL core material in the first year and the AHL material in the 2nd year. This is not a very realistic scheme unless there are special timetable requirements, or SL students use time in the second year for something specific. It would leave the SL students with little to do in the 2nd year and it would give a long gap between studying the biology and taking the exam.

SL students are commonly taught in a mixed SL / HL class and I have used both the models above. Encouraging these students to come to all classes and when the AHL material comes along let the students leave with work to do. For example; to work on their lab reports, to study for an upcoming test or to catch up on something they have missed. This does demand a certain amount of self discipline on their part but as they are young adults, they need opportunities to learn independent study skills some time!