I don't think I can add much new to the discussion that hasn't been said already, but for me it would definitely be Arrival because of story reasons. However, it is one of the very few missions in the game that you go solo and it depends on whether that's your thing. I love the companions and BW games are largely about them for me, but I enjoyed having one big mission just for "myself", as well. The gameplay is quite varied - it actually offers some stealth elements, for example, which was really odd, but surprisingly enjoyable. I think the pacing is quite intense which is something I liked, as well. Ultimately, it is a link between ME2 and ME3. Your ME3 experience will be fine without it, but I liked having a mission that was actually relevant to the overarching plot.
I love the Lair of the Shadow Broker, as well, and it also explains why some things are the way they are in both ME2 and ME3, but like Arrival, it is not 100% essential. I think the gameplay is great, there's tons of content, and, well, it has Liara, if that's a deciding factor. (I really like Liara in this one, personally.) But, well, you have that one, so you'll see.
But, well, it depends on what you like. I didn't like most of the armours and weapons in the base game that much, besides rare exceptions, and felt there were very few, so I bought the Firepower Pack (which has a really great assault rifle) and Aegis Pack (which has a nice armour). I don't like buying these things as DLC usually, but I made an exception for these two. I also got Kasumi which has a truly fun and enjoyable mission + plus a charming companion, but it doesn't have anything to do with the plot. But then again, it adds one of the best weapons in the game. Overlord was pretty amazing, as well, storywise. The gameplay parts are going to be a matter of taste. I found them mostly fun, but I can't say I'm a big fan of the Hammerhead. Still, the level design was pretty awesome.
Definitely get the free DLCs like Zaeed because, well, MORE CONTENT! And Zaeed is pretty awesome. Even if you don't use him, it's still good to have them as options. I, personally, also liked the Crash Site a lot.
For those who have trouble with how some of the squadmates dress, which is sadly a fairly legit complaint, or just want more options, there are Alternate Appearance Packs.