From my experience, the light vanilla shake is best. I've come to appreciate the flavour more and more over time. It's subtle, has different enjoyable hints and is very balanced - no particular flavour drowns out the others and the texture is very smooth at 350 ml water. Close to perfect.
I can appreciate what they've tried with forest fruit. It's not too sweet and kind of sour, which also holds true for actual fruit. However, a lot of bad flavours shine through. Chemical flavouring, bitter hints, a grittier texture and an aftertaste that doesn't really match the shake.
Banana is better but in many ways the same as forest fruit. Also, vanilla works nicely in combination with fruit, coffee and cacao, which is a plus. Vanilla + a banana > Banana. Whilst banana and forest fruit don't seem to mix as well.