I heard Layer of The Shadow Broker is good, but I have also heard Brad say Overlord is the best. What do you guys think? I also guess there is Kasumi- Stolen Memory. 

My vote goes to Shadow Broker. I'd consider Overlord to be nearly equal except for that fact you have to drive around in the tin foil tank.

Shadow Broker, but they're both worth getting if you can afford it.

Shadow Broker

I haven't played through Shadow Broker yet, but Overlord's ending was absolutely incredible. I don't see how Shadow Broker could do better than that.

Shadow Broker, Overlord's story is a self-contained side-story, Shadow Broker's story is HUGE for Mass Effect 3 and the whole Mass Effect universe.

Lair of the Shadow Broker and Overload are both great, but if you can only get one I'd say go with Lair of the Shadow Broker. It's the longest of the bunch and easily the most entertaining. As for Stolen Memory, it's alright, but you're not missing much if you don't get it.

Shadow Broker, hands down. 
Overlord has an interesting story but its more of a side story. 
Shadow Broker's story can be part of the larger Mass Effect story and there are many easter eggs for fans of the story.  
If it was purely for gameplay, probably overlord but mass effect 2's story is best augmented with shadow broker.

I preferred Shadow Broker to Overlord.

I would personally say Shadow Broker myself.  I hear Overlord is fun, but the story in Shadow Broker was more relevant to the game than Overlord in my opinion.  It may also have a more significant impact as to what will happen in Mass Effect 3 than Overlord.  Someone should correct me on the last statement if there is some crazy story-related thing that may be just as or more impactful.

Wait, I thought that once Mass Effect 2 came out on PS3, you guys got all that DLC free too?
 
Anywho. The Shadow Broker DLC was freaking awesome. Best out of the DLC by far. Kasumi is fun and all... But there's a little bit of story, then a long and overdrawn fight session all the way until the end. It got really boring and I, myself, would never pay for that additionally.
 
Overlord on the otherhand... Well, I'm indifferent to that. I didn't think it was that special, but it was definitely not bad.

" Shadow Broker, Overlord's story is a self-contained side-story, Shadow Broker's story is HUGE for Mass Effect 3 and the whole Mass Effect universe. "

I enjoyed them all. But Shadow Broker is my favorite because it relates more to the overall story.

Shadow Broker. Made me more happy with sticking to Liara on my main Shepard.

I thought Overlord was the best when taken in the context of a single mission, but I would recommend Shadow Broker over it because it is much more important to the larger fiction.

Shadow Broker is more key than Overlord story wise. Plus Liara.

I haven't finished overlord, but so far I like it better than Shadow Broker.  

1. Shadow Broker 
2. Overlord 
3. Kasumi 

Don't listen to Brad, he wouldn't know good DLC if it bit him in the ass. Shadow Broker >>> Overlord >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> Minerva's Den

You get more content from playing Shadow Broker than you do for Overlord, so I say go with that. Brad's a sucker for story, and I can be too sometimes, but that doesn't make Overlord better.

Shadow Broker is better than the other two combined. 

Shadow Broker, it's the only one that really has an impact on the overall narrative. I would be very surprised if events that occur within that DLC aren't mentioned again in ME3, they seem too significant to be brushed aside in my opinion. There is also some very nice easter eggs packed in there as well, if you are invested in the universe buying this is an obvious choice.
 
Overlord and Kasumi are both good but are standalone missions and stories, they definitely feel seperate. That said I found the story elements of Overlord were excellent.

Shadow Broker. Can't get enough of Liara!

1. Shadow Broker 
2. Overlord 
3. Kasuma 
 
However, if you are starting a new playthrough I would get Kasumi then have her in your squad to experience her unique dialog and reactions while on missions.

Shadow Broker is a good set-up to Mass Effect 3 but I personally enjoyed Project Overlord a bit more. It was a self contained story with a harrowing conclusion. :x

shadow broker, since its story is actually kinda essential to the main plot, whereas the story of overlord is kind of an isolated thing not related to anything else

Definitely the Shadow Broker.  

Overlord is OK but it's very much a side story and (in my opinion) much of the drama is lost because of how short the story is and how little time you have to get to know the characters and understand what's going on.  Shadow Broker is much more epic and continues storylines from both ME1 and 2.

Shadow Broker and Overlord.
 
Rest meh, Kasumi's mission is fun but not worth the price tag, they had a sale a while ago and even with that I felt a lil bad about the content versus the price.
Not sure how she's holding up as a squaddie in the story if you take her along tho.
 
I''d say Shadow and Overlord, and save your cash for the coming DLC or ME3.