Citadel is a must. If you buy no other DLC, buy Citadel.
Citadel + From Ashes would be the best bang for your buck in terms of quality.
Leviathan gives you backstory on the Reapers. It's interesting, particularly if you're a lore nut, but its impact on the game is slim to none, and the content itself is fairly short.
Omega is probably the worst of the ME3 DLC. I enjoyed it, but the story was pretty lackluster and short. I'd only recommend it if you're a huge fan of Aria and/or Omega.
From Ashes is a great buy because you get a permanent squad member out of it. He, of course, gives you more insight into the Protheans, and his interactions with the game add a great deal of additional things to see as you go through the game. Javik turned out to be one of my favorite squad members in the trilogy.
Citadel is, hands down, the best DLC in the entire trilogy. Some have even gone so far as to say the best DLC for any game ever made. The story is interesting, the humor is incredible (it was rare that there was a time in this DLC that I was not laughing), and you get a massive amount of additional game time from it. As I said above, if you buy none of the other DLC, you need to buy this one. You won't regret it. It's my favorite DLC by far.