Which is in your opinion the order to play the DLC´s on ME3?

Well I don't have Leviathan but I always do Omega after Citadel II. It always fit better for me there. Citadel dlc I do right before the end.

From Ashes after Menae, or after the Citadel coup if I want Kaidan's/Ash's dialogue.

Leviathan before meeting the asari Councilor pre-Thessia (After Thessia Traynor tracks Leng, so I'd go right after him at this point).

Citadel before Cronos Station, because the Normandy needs to be shiny and ready.

Omega... never

Here is how I usually do it:

If new game (import from ME2):
1. From Ashes after gaining control of the Normandy (and planet scanning)
2. Omega after The Shroud
3. Leviathan and Citadel (main missions) after Rannoch arc/before Thessia. I may do these sooner depending on how soon I want the weapon mods and/or if I do not care about having Tali for these missions.

If New Game+ (import from finished ME3 playthrough), typically:
1. Beat Ardat Yakshi Monastery first, research intel to get +10% store discount.
2. Omega before meeting
3. From Ashes before meeting with Quarians
4. Leviathan and Citadel (main missins) before meeting with the Quarians, or after Rannoch arc/before Thessia.

A lot of people say do From Ashes as soon as it's available to get the most dialogue out of Javik. However, I like the extra dialogue from the Virmire Survivor. Kaidan was there from the start and Ashley was recruited on Eden Prime. Either will have special dialogue on the shuttle ride down (assuming you didn't kill them in the coup). I like that the best. So I leave this until after the Coup.

Omega after Coup and after Aria's quests to recruit mercs (makes more sense since, in theory, Aria would be returning to Omega).

Leviathan can fit in a lot of places but I generally prefer to have all of my squadmates to choose from before going. Definitely make sure my LI is recruited if a squadmate. Special LI dialogue at certain parts. Generally do this after recruiting Tali.

Citadel immediately before Cronus. Gives me the most options for people to attend party and meetups with friends. I'm pretty sure Miranda can't appear until after her quest is done and that's pretty late in the game.

From Ashes DLC - After Citadel Coup and Kaidan is recruited. I like how Kaidan had a small talk with Shepard about ME1 and by then Javik would sense Shepard's truth about the state of war and I like "confident" dialogue. After that I finish Rannoch arc, I put off meeting with the Asari Councilor and finish up Tuchanka's Turian Platoon because I like how Ashley's death was referred by Tali and Kaidan.

Omega DLC - Right after ^ and other N7 missions, made sense since it lead up to you trying to take down Cerberus and finishing the Crucible as it need Eezo resources... and immediately afterward..

Citadel DLC Part I (Conspiracy) - of course the Normandy team was having a some shore leave while Shepard escape to Omega and it made sense for Normandy to restart and finish the retrofits. I don't think the forced depression mood is suitable for Citadel's light tone. And its nice to see Shepard and Kaidan having some time together after their years of separation, the behave like normal couple for once.

Leviathan DLC Part 1 (Dr Bryson and Mahavid) - I usually put this DLC off until after Horizon but nowadays I start the whole Leviathan intrigue earlier to set the tone to Thessia... also speak to Asari Councilor here.

Leviathan DLC Part 2 & 3 (Namakli and Despoina) - I break these two sometimes, after Thessia (Got out of a reaper infested planet with a depressed xenoarcheologist and find another xenoarcheologist in another reaper infested site) and after Horizon (you go to Citadel, talk to Ann and track Leviathan). Most of the time I just do them in one swoop after Horizon.

Citadel Part 2 (Party) - Normally I do some of the hangouts in between, I usually save dinner date with Kaidan before starting the party. Kaidan cook steak for Shepard and they had wild sex in the living room and after that she started the party which they later have drunken sex again... and then they went back to the ship to end Cerberus and they had sex again because he is an overachiever that's why.

From Ashes I do right away, either right after Menae or even before (though that's not really worth it as it only gives you one single additional conversation with Javik).

I usually do Omega after the coup (like most people here apparently).

I do Leviathan after I am done with Rannoch since it seems there is some time there.

I either do Citadel right after Leviathan or just before going to Cronos Station. In the end, I think it works a bit better before Thessia because I feel like there is too much of an instant change in moods otherwise.

If I use ending mods, I might also use CEM (usually the light version, where I can do the actual Citadel missions before the ending but then have the party afterwards).

From Ashes - I pick up Javik early on, so after Menae. I like assembling the crew as a 'first things first' kind of thing.

Omega - After the Coup seems reasonable - retaliation on Cerberus for the attack on the Citadel, plus I figure Aria figuring that this is the right time to strike.

Citadel story - After Rannoch seems like the most reasonable point for the goofy screwball comedy this is, since morale would be high after resolving the geth-quarian conflict, and Normandy actually does see some action in the fight.

Leviathan - After Thessia/Horizon. Since Leviathan does actually explore the origins of the Reapers, having Vendetta bring up the uncertain nature of the cycles, the fact that the reasons why are unknown, without bringing up what Shepard knows of the Leviathans just seems weird. I kinda flip on which of them it works better after, since Thessia, Horizon, and the endgame all seem to lead in to one another, so sometimes I say after Thessia, sometimes after Horizon.

Since I'm in an ME3 pt right now, and I saved Citadel for after Horizon, I think I'm going to make some changes. I'll start it earlier, like after Rannoch. There is too much of a sense of urgency once you're on the trail of Kai Leng, which you are immediately after Thessia. I'll still wait on the party as the last thing they do before running off to their fate at Cronus Station and then to Earth.

I play from ashes after the coup, with Kaidan & Liara, like the banters with the 2 of them, "recalling" old memories

Leviathan after Thessia, to cheer up Shepard & give some hope

Citadel well depends. When MEHEM is installed, I'll play it after I finish the game, so it feel like it's the last goodbye. Without MEHEM I would play it after horizon, I imagine they are making some last repair & final check up while the fleet is gathering.

Didn't buy Omega, still waiting for a price drop on origin, not gonna spend 12€ in the lazy dlc :x

From Ashes
- After Rannoch, if I want Tali and Liara in the squad.
- After Citadel coup, if I want Liara and Ashley or Kaidan in the squad.
- Otherwise, after Palaven.

Omega
Anytime after recruiting the Blood Pack, Blue Suns and Eclipse for Aria. (I usually do it after the Citadel coup).

Leviathan
After Sanctuary. I know lots of players like to spread the different stages out throughout the game, but I like doing it as one of the last things near the end, as one long mission. It fits best story wise (in my opinion), and avoids “spoilers” like encountering banshees before you run into them on Thessia or the monastery.

Citadel
- Main story - after Rannoch. All the squad-mates have been recruited, and you've had a few “victories” (Tuchanka, Citadel coup, Rannoch) that you can celebrate with a more light-hearted mission (although they may not feel like victories to you depending on your choices and the outcomes).
- Meet-ups, Armax Arena quests, etc - stretched out over the rest of the game. I'll do a mission or two, then come back to the Citadel for whichever squad meet-ups have unlocked and an Arena match or three.
- Party - last thing before Chronos Station. All of the former squad-mates have been encountered and the meet-ups are done, everything else is finished and I want to have one last get together before launching endgame.