Personally I think the Original powder works best as it’s a more neutral flavour.
The vanilla has a strong toffee flavour which does work quite well with some of the boosts (mint, mocha), but doesn’t work at all with others (matcha, pineapple, caramel)
If you prefer it less sweet then the unflavoured works really well with the flavour boosts but you have to use much more boost so the sample sachets aren’t quite strong enough unless you make a mini 1 scoop shake.
I find the perfect blend for most of the flavour boosts is 50/50 Unflavoured/Original.
The best flavours in my opinion are mocha, caramel, Cacao, and matcha
Edit: oh and mint choc!!! The boost is quite different from the premix mint choc. I love both. The premix is more refreshing and like buttery mint with a hint of chocolate, the boost is more chocolatey and also minty.
Definitely Original rather than Vanilla, and I also dilute the sweetness of Original with Unflavoured, though more like a third Original to two thirds Unflavoured these days.
I find Original far sweeter than Vanilla. So much so that I have to mix it with UU to be able to drink it. 25g Original to 75g UU. Vanilla I can drink as it is.