Leviathan should have been a part of the game. It unveils some pretty important parts of the main story, so that's mu vote.
Citadel is kinda like a celebration. Thank you for playing Mass Effect, here is one last great DLC, with some game modes added on. It's the best DLC, but it's more a fan-game. It's kinda like an epilogue, but not really, and it's pretty funny if you played all 3 Mass Effect games.
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Omega is pretty much useless. It doesn't add to the story, isn't particularly interesting, and I didn't much care for it.
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So it depends, if you want more story, Leviathan is your best call. If you want a DLC which plays kinda like a "Bonus Features" to the game, with a blooper real, Citadel is your choice. IMO get both, Leviathan is pretty cheap, and it's pretty old too so it might be available for sale soon.
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