Nutrition-wise they are all identical.
The only difference is that by having pre-mixed flavours eg Vanilla or Berry, you are consuming sucralose and flavourings. Some people believe these are ‘unhealthy’, but the science doesn’t seem to back this up. I personally react adversely to chemicals and additives but get on fine with flavoured Huel. If you want to read up on it, the Huel website has some articles on the ingredients including why they chose sucralose and why it doesn’t have the same negative health effects as other sweetners.
The flavour boosts contain Stevia rather than sucralose, so if you prefer natural sweeteners, the UU plus flavour boosts is a better option.
To put simply - they are all healthy. None contain anything bad for you. And the nutrition in all is the same.
So it’s just a matter of personal preference as to whether you want to avoid all sweeteners.
If you do want to stick to UU, it is easily flavoured using instant coffee, cacao powder, fruit etc - search the forum for lots of recipe ideas!
Also, just to clarify: Vanilla isn’t the same as Unflavoured. Unflavoured has no sweetener, Vanilla has sucralose.
If you want to avoid sucralose, then Unflavoured plus flavour boosts is better.
But, if you are getting on fine with Vanilla, then you are obviously not having any adverse reaction to sucralose, so you might as well enjoy the other premixes as well 
And try the flavour boosts!
The caramel, mocha, cacao, and mint-choc are delicious 