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.
