If you were going to get any DLC at all (not incl EC), I'd say Leviathan and From Ashes b/c they flesh out "the past" more. That said, they don't change the RGB ending a lick, so really aren't essential to figuring out the overall story. Haven't played Citadel, but hear it's nothing more than a complete distraction from the overall story (albeit a fun one, apparently). Frankly, outside of maybe adding crewmates, none of the DLC in any of the 3 games is really essential to the plotline.
Now, I want you to remember that no bastard ever won a war by dying for his country. He won it by making the other poor, dumb bastard die for his country.---Patton
There is no room in this country for hyphenated Americanism.---Teddy Roosevelt
I never forget a face, but in your case I'll make an exception.---Groucho
And if you all get killed, I'll piss on your graves.---Shaman Urdnot
How would you like to own a little bit of my foot in your ass.---Red Foreman