Divine Divinity and Beyond Divinity should be played in order.
For Divinity 2, the Developer's Cut is a slightly updated version of The Dragon Knight Saga (a few extra fixes), with an added Developer Mode cheat window (which can be fun to play around with, even if you don't want to actually cheat in the game). The dialogue and books in the game give an overview of the first 2 games, primarily what happened between them (a novella included with BD) and the end of BD. Lots of people did play Divinity 2 first, though, and then go back and play the first game(s); there aren't really any spoilers for DD.
Dragon Commander takes place thousands of years before the other games, so is unconnected in plot or geography. It can be played at any point before, during or after the other games.
Original Sin is set 1200 years before DD. It kind of leads into the backstory of DD, but isn't directly connected to the later games. If you have played DD first you will recognize a couple easter egg type references and be more familiar with the Divinity lore, but it isn't anything essential to be able to play the game. You can play this at any point between the other games.
I'd recommend playing the first 3 in order, but if you really wanted to start with D2:DC you could. DC and D:OS can be played in any order. Are you in the mood for any particular style of gameplay, RPG vs RTS/RPGish, isometric vs third person, real time with pause vs turn based, single character vs party?
