Shadow Broker is the best DLC in terms of gameplay content. It and Arrival are the most plot-important ones; Arrival isn't really particularly interesting in terms of its levels etc, but it fills a gap in the story between 2 and 3 (ME3 more or less assumes you've played Arrival) so if you care about that, it's worth a pick. The NPC ones aren't much worth bothering with, the game's already got plenty to choose from and the DLC NPCs don't really interact with you/other team mates beyond their own missions; on the plus side, they also add a tiny bit of ME3 content (glorified cameos, but still). Overlord I've not played. There's also the Normandy wreck DLC which is rather lovely in its own way, but it's a VERY small piece of content.