What milk and yoghurt is the best for mango lassi?
It all depends on taste [of course].

Lower fat milk &amp; yoghurt are sharper, more tangy, almost 'lemony'.
High fat are rich, smooth &amp; creamy.

So, start with 'How tart do you like your lassi?' &amp; work from there.

Personally, I like lassi to have some 'bite' to it, so I'd go for zero-fat yoghurt &amp; probably what in the UK would be called semi-skimmed milk, around 2% fat - but that may have more to do with it being the one we always have in the fridge than any other consideration.

I'd say if you want a 'smoother' feel to it, increase fat content on both, but be prepared to have to thin it down further to achieve a drinkable consistency.

BTW, don't use set yoghurt ;)

Yoghurt itself varies in acidity between varieties even at the same fat content, so you have the additional task of figuring out which brand gives the best start-point.
I can't really recommend any particular brand as I doubt they sell my usual Lancashire Farm in Poland ;) but I'd always pick a large independent who look like they might actually care over one of the major multinationals.
An additional factor is that you can now get zero fat Greek yoghurt… which completely breaks the standard pattern of lower-fat == more acidic.