I thought Arrival packed a pretty decent punch, even though it was one of the weaker DLC's. Good setup for ME3, so well worth it at that price.

For me, I'd rank the story DLC as follows:

Shadow Broker, Kasumi (for the SMG and Kasumi is a BAMF character), Overlord, Arrival, Firewalker.

Not counting Zaeed or any of the DLC that came as part of the game's release (ie the Normandy dog tags, which I personally thought were one of the game's most underrated missions)