Play them all! I loved both DD and D2 for albeit different reasons. D2 was my first venture into the series, I finally played DD very much later but in spite of its by then not at all up-to-date graphics I had a really fun time with it.
The one I could never really get on with is Beyond Divinity. Which is nothing to do with the gameplay, but some stuff felt a bit off with the presentation, which probably sounds a bit shallow but I could never get into it as much as the other games in the series.
Actually the gameplay in Beyond Divinity is also not the best. Offensive magic spells are essentially unusable thanks to the exorbitant mana cost, and it can be awkward controlling two party members in real-time.
