From Ashes - Technically can do any time, but better for the first half of the game (before Coup), and ideal for right after Manae for best story access and narrative flow. That said, it is kinda weird to go to Eden Prime so early in the game, so you may wish to take a bit longer. Depends on how much exclusive DLC lines (like post-missions) you're okay with missing.
Me? I like to do it right before Manae, getting full access asap (just as I'd recruit Zaeed and Kasumi asap).
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Leviathan - It feels like a rather endgame storyline, so while it is available rather early, I'd suggest not doing it for a while.
First mission --> After Rannoch
Second mission --> Before or After Thessia (I prefer after)
Third mission --> After Thessia
Or, since I like to do them all at once as its own 'arc', I do all three missions as a chain right after Thessia or Sanctuary. For more details of how this section of the game works with DLC, look at the Citadel DLC section.
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Omega - It has elements that works better with various parts of the game. It works with early game due to meeting Aria early. It works with late game due to the enemy selection and the feeling of pressing upon Cerberus as you approach the end. But IMO, ideally, it works best RIGHT after the Citadel Coup. There is a gap in the story that you can fill with this DLC, and it lets you get 'revenge' against Cerberus for their attack.
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Citadel - This is divided in a few sections: Missions, Visits/Casino/Arcade/Arena, Party. Technically, you could do them all at once, with arena matches counting as a 'mission' that unlocks more visits with characters. No matter what, it is best to save the party itself for the VERY last. If you wish to do it all at once, I'd say do it all as your last activities before Cronos Station (so after Sanctuary and/or Leviathan). But if you wish to spread it out more, I like do go:
Missions --> Between Rannoch and when the story gets more serious on Thessia, since this mission-line is the most comic of anything in the game (even the Party section itself). Or if Tali is dead or you don't want her involved, you can do it right before Geth Dreadnaught.
Visits/Casino/Arcade/Arena --> Between missions as you progress through Thessia-Sanctuary. Visit the Silversun Strip once or twice, and complete activities there, and if you want to unlock more visits, just do an Arena match (and remember the sidemissions for the Arena!). Basically, treat the place as Shepard's relaxation hub during a time when things are getting more tense. Though it might be smart to chain Thessia and Sanctuary together, as they have a direct relation (tracking Cerberus). If you did the Mission section BEFORE the Geth-Quarian arc (due to not having/wanting Tali), then that gives you even more room to mess around.
Party -->
1)Do it as your very last action ever, before Cronos (including ALL sidemissions EVERYWHERE).
2)Again, last action, so have Sanctuary done, *especially* if Miranda is set to live. And make sure that if she's alive, she got her Visits done.
3)Try to pace things as if Shepard got the coordinates of Cronos Station, is almost ready to go, but has the time for just a day or two of final meetings/goodbyes in a party with everyone. That is, the bulk of the time consuming and plot interrupting sidemissions ought to be finished before Thessia if possible (a few are not possible until later on though).
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MY PERSONAL ORDER FOR MY MAIN SHEPARD
-Vancouver
-Mars
-From Ashes DLC
-Manae
-Sur'kesh
-Grissom Academy
-Turian Platoon
-Cerberus Bomb
-Rachni Planet
-Tuchanka
-Citadel Coup
-Omega DLC (4 missions)
-Asari Monastery
-Ex-Cerberus Scientists
-Geth Dreadnaught
-Quarian Admiral
-Geth Consensus
-Rannoch
-Citadel DLC (Missions) (4 missions)
-Thessia
-Sanctuary
-Leviathan DLC (3 missions)
-Citadel DLC (Party)
-Cronos
-London
-Ending
Obviously, Visits etc happen between the Mission and Party section. They may even happen between Leviathan missions or N7 or other Sidequest excursions. Especially since Shepard is hanging around the Citadel for Leviathan, and therefore ready to spend just a few hours in an arena to train, or have a little bit of fun with a friend.
HOWEVER, I do typically chain the last two Leviathan missions together, without interruption. They seem much more tied together than the first one is to the second.
I tend to see the Thessia-Sanctuary/Leviathan/Citadel area of the story as the section when 'things' are getting more serious and deep (I suppose in a way), while Shepard also has the relatively most free time. I don't think this is 'right' to do (EARTH BE BURNING), but I think it fits the story enough as we've shown it. Of course personally, I have my headcanon(?) that Shepard's getting more and more sucked into a mind realm anyway, thus scenes like with Kaidan where the TV screen is showing the Earth attack from Vancouver mission and Shepard doesn't even blink at it. But aside from that, it does make some minimal sense that this would be the point where Shepard and co recharge the most at the Citadel and try to prepare themselves the most for the biggest fight to retake Earth. I mean, I believe it was confirmed that the general conception of the Citadel DLC was actually a DLC idea for ME2 (thus maybe a pre or post Suicide Mission rest and relaxation DLC there as well, despite the also dire countdown going on there, with colonies being taken).