I find the vanilla flavour overpowers some of the flavour boosts. Most of the flavour boosts work better with the original powder. Also you only get 2g in the sample sachets, and I tend to use closer to 4g of flavour boost for a 3 scoop shake. I thought the cacao fs was quite nice, but real cacao powder was nicer… so not really seeing the point of it, unless you want extra sweetness (as the cacao fs has stevia added whereas raw cacao powder is just cacao). The flavour of the cacao fs is slightly different than just cacao powder - I guess it just depends on your preference. The chocolate fs is quite nice too - i don’t know if you’ve tried it? Its more like proper ‘chocolate’. Again though, I would use the flavour sample with a 2 scoop shake to get a more intense flavour, to see if you like it.
All the flavour boosts work better when you have a full bag, as you can add as much or as little as you like, to your own taste.
My personal favourites are Mocha, Caramel, and Mint-Choc.
Edit: I always have three bags on the go (well four if you include the new yummy mint-choc premix): U/U to mix with Original and / or vanilla to reduce the sweetness. Vanilla is nice to drink on its own, and Original definitely mixes better with the flavour boosts as it has a less intense taste, so the added flavour comes through better.
So, if you find your shakes too sweet, get a bag of unflavoured to mix with, and if you prefer flavoured shakes, try out a bag of original and see if that works out better for you.
2nd Edit - the Original powder has a more intense sweetener taste… so actually works better with raw cacao than the cacao fs in my opinion. Original powder plus instant coffee is also awesome