Personally, for introduction to divinity games, I recommend playing a Divine Divinity (Divinity : Sword of lies). It is a classic game, big world, a lot of fun quests and good story.
Divinity : Original Sin 2 is an obvious choice, best game in divinity series so far. If Larian would restore all cut content/characters for it in some patch, I will say it a perfect game. After all, if CD Projekt Red is making a DLC for 11 year old The Witcher game, while making a new one, which also is increasing hype for it, why Larian can't do the same?
