The free Hammerhead DLC was kinda bland, Overlord was pretty cool (reminded me of a much better version of an ME1 subquest), but Kasumi and Shadow Broker were my favorites. Shadow Broker due to plot reveals, and I loved the scenery of the mansion in Kasumi's, and the whole "infiltrating the party" bit.
I have every ME2 achievement so far except the "Beat the game on Insane". Will be getting the Arrival DLC on release, shame it's a short one but hopefully it will be priced accordingly.