Origins is the first game, and will definitely serve as the better introduction to the setting and series, so if you're looking for a place to start, that's it.

Don't listen to the people bashing DA2 though. If you like the series when you try Origins, give it a try too. You may find that, like me, you actually consider it superior to its predecessor.

There's also Awakening, which is a DLC expansion for Origins set after its main plot. Worth getting if you like Origins (play after Origins, before 2), but not really as good as either of the main games. Introduces one interesting character who could make a great plot hook later on, and that's about it when you get down to it.