Divinity Original Sin 2 would be my vote.

The BG series were great games at the time. They are the first games I ever bought for myself...however they are very very dated and DnD didn't translate perfectly. If you love the genre and want to see the classic then they might be interesting to play but more modern cRPGs are on the whole better and if new to the genre you would be better off playing something like Pillars of Eternity (edit: PoE would be my choice if you want story heavy and a by large more serious tone than D:OS though mechanics are not as interesting) , Dragon Age: Origins or Pathfinder. DoS:2 is less "classic" or "traditional" but that's not a bad thing at all, it modernized and improved on mechanics in ways that make sense and it encourages experimenting (and easy respec and simple character creation means no front loaded build crafting and you don't have research shit before playing which is a massive plus) though as result it plays differently to most.

I don't think you will need to play classic BG to understand the upcoming BG3 though it has a few recurring characters. If you decide to play BG2 keep in mind that it is a direct sequel to BG1.