I've never been really a huge fan of ME2 DLCs, the lack of squad chatter just makes them feel unattached from the rest of the game and for me the characters were the charm of ME2. If you're going to get just one DLC, it has to be Lair of the Shadow Broker. It's by far the one with the most effort put into it and despite the lack of new lines for characaters, it truly gave some characters an extra twist, for example made a certain Cerberus ***** tolerable on at least some degree. Get it, play it, complete it, you'll understand what I mean. Also, it has Liara, who you might not know that well if you didn't play ME1. If you did, good for you.
Kasumi to me wasn't a terribly interesting character, but a fine addition to your crew. Her loyalty mission was alright and rather differs from the others, at parts I felt like I was in a James Bond -film but yet didn't really feel out of place.
Zaeed is an old mercenary with some rather amusing one-liners, his loyalty mission is a bit more traditional ME2, it's solid but not groundbreaking. If you're just looking for more ME2 to play, this one is a fair deal.
Overlord is an alright DLC too, mute teammates ruin it a bit for me but it has some solid gameplay and the ending is one of the best moments of ME2 for me. Don't spoil the ending, play it yourself. This one was a bit crash-prone though, and it seems having Legion in your squad causes some crashes, so don't take him with you.
Arrival is by far the worst DLC, it's the shortest and yet the least interesting to play through. Instead of mute teammates, you have no teammates at all for no good reason, and it just feels like BioWare tried to come up with a good explanation why they didn't hire the voice actors to do more lines. This one should be the most significant one story-wise, but yet there is no sense of hurry, it's like "Yo, my friend has been captured with batarians, she has new information on the Reapers but go there when you feel like it, let's say after you've shagged your whole squad". I'd recommend you not to buy it and if you feel like you're missing out story-wise, watch the cutscenes on Youtube.
LotSB > Kasumi = Overlord > Zaeed > new outfits for some of your squadmates > Arrival.