1. From Ashes - A MUST. Nuff said. Don't play ME3 without it. Javik is an essential ME's character,to the point where BW even considers him canon.
2. Leviathan - Adds additional backstory to the Reapers and one of the most well-written dialogues in the trilogy (no spoilers). Plus,on top of that,you get an excellent story,very good characters,and a few tense combat situations. Definitely worth the money. Plus,if you're a perfectionist it gives you a biggest War Asset in the game (again,no spoilers).
3. Omega - After hearing all the complaints about Omega,when I first played through it I was very pleasantly surprised.
Don't listen to what people say,Omega is one of the highest quality DLC's in the trilogy. Its very well written,has plenty of fun combat,good soundtrack,and a (at least in my opinion) very interesting story. Essentially its retaking the entire Omega back from the Cerberus. You'll visit previous locations from Mass Effect 2,and Aria will even make references to Mordin,to which Shepard will respond depending on Mordin's status in you're save file. Honestly I don't understand why people bash Omega so much.
Sure,its 2 and half hours long,but in Omega's case its quality over quantity. Pick it up,you won't be disappointed.
4. Citadel - Citadel is a DLC for hardcore ME fans. I'd recommend it only if you have a strong attachment to the characters,your LI,and the universe overall. Its full of funny references,awesome ME-type moments,etc. But as I said,only if you have a perfect playthrough - that is,all squadmates alive,etc.
Otherwise you might come out of the DLC thinking "WTF did I just play through?".