MISSION DLCs
Normandy Crash Site - Normally i wouldn't recommend that you get this, but since it's free, there's really no reason not to. It's not that long, and mostly just there for the nostalgia factor.
Zaeed - Adds a new loyalty mission, a flamethrower heavy weapon and (most importantly) A new squadmate, a pure combat specialist. This one is also free, so get it.
Firewalker - Boring. 5 levels of piloting the Hammerhead through boring environments. This one is also free, so it's up to you. you're better off sticking with overlord to get your hovertank fix.
Kasumi - 1 new loyalty mission with a really fun James Bond atmosphere, 1 great new SMG, A new casual outfit (the female version of which is unique) and 1 new squadmate, a tech specialist with a focus on stealth abilities. This one is around $7.
Overlord - A well-written plot that has some background links to Cerberus (with an amazing ending) and Hammerhead sections that are actually fun compared to Firewalker, It's also fairly lengthy. This one is also around $7.
Lair of the Shadow Broker - If you get only one DLC pack, It has to be this. This is not only the best DLC mission, It's one of the best missions in the entire game. This one is centered around Liara and reveals the identity of the Shadow Broker that you've heard so mush about in the past 2 games. It is considered canon, so you're missing out on a good portion of the story if you don't get it. This is around $10.
Arrival - This one is jut "meh." It feels rushed, like it's only there because the writers backed themselves into a corner and needed to set up the beginning of ME3. There's barely any dialogue and the one major choice in the mission is made for you. You do get to meet Admiral Hackett in person, though. The plot is predictable and contradicts a lot of the story of ME2. Oh, and it's a solo mission, because they didn't feel like giving an excuse as to why you're squadmates don't talk for the entire mission because they didn't hire the voice actors. Around $7.
WEAPON AND ARMOR PACKS. I'm not going to list all of these, just the ones that are somewhat interesting.
Cerberus armor and weapon pack. - It's free, so get it anyway. One suit of armor that you will probably never wear. The real item of interest is the shotgun, which you get at the beginning of the game and outclasses the starter shotgun in every way.\\
Arc projector - This is also free. A heavy weapon that shoots lightning and can jump from one enemy to another. That's really all there is.
Aegis pack - 1 amazing burst-fire sniper rifle along with the only armor set worth buying. $2
Firepower pack - 3 great guns. A semi-auto assault rifle that does a lot of damage per shot, a pistol with a laser sight that's slow-firing but accurate and powerful, and a shotgun that works a lot better at medium range and can be charged for extra damage. $2
Appearance pack 1 - 1 new outfit for Thane, Garrus, and Jack. Thane's outfit is a bad re-skin with stupid glasses and Garrus gets another re-skin with the gaping hole fixed and a new microphone, along with some impractical looking shiny neon lights. Jack is the one this pack benefits the most, as she finally gets some clothes. unfortnately, she also gets a stupid looking visor. $2
Appearance pack 2 - same thing with Grunt, Tali, and Miranda. Grunt and Tali's outfits can be ignored, as they're just re-skins, but Miranda actually gets some armor so she doesn't prance around into battle in spandex and high heels anymore. $2