A couple months ago I would have said breast and fly because of the undulation. Recently, however, I have really started working on keeping my body line as clean as possible in free and back and find that just as intense, if not more so.
Try this, Stand in a full length mirror and "swim" as if the mirror is the bottom of the pool. You can do free or back. Now try to maximize your range of motion without allowing your torso to bend in any way. You can't wear loose clothing or you won't see your bends.
Next, lay on the floor, on your back in streamline position. Try to get your hands, shoulders, low back, and knees on the floor. It takes a lot of ab strength (and shoulder/hip flexibility) to do it.
I started thinking about this when an interviewer asked Dara Torres what she thinks about when racing her 50 and her response was "tone". She later clarified that she meant keeping her body just firm enough that everything stays connected and straight.