Best Mass Effect level design

I think Dossier: The Assassin is my favorite mission out of any ME game, level design wise.

Was that the Dantius Towers mission?

I remember working my way up there...

If we draw a veil over the Hinterlands,
I liked the open space of the Frostback Basin (Jaws of Hakkon) for its variety and verticality.

I liked Thane's recruitment and the Illium section (Baria Frontiers, before Azure section) the most. Virmire was great too.

I like 'new Firebase White' with the connecting corridor (the patch to stop Gold farming).

And of the multiplayer maps, Hydra is probably my favorite...

Thane's recruitment mission is great, but I think my favorite is actually Grunt's recruitment mission on Korlus. In ME3, I think my favorite is the Leviathan mission on Namakii.  In ME1, the top spot goes to the Ilos/Citadel end mission.

ME1 - Bring down the sky
ME2 - collector ship
ME3 - Tuchanka - curing/sabotaging the genophage

 
I found BDtS to certianly be a step up from the "average" warehouse experience of ME1, but the overall flow of the whole DLC seemed rather "choppy" to me and the end bulding battle can get rather buggy.  Several times I've had to reload it and do it over to get Balak to trigger.  I find the Ilos/Citadel end mission flows very well.  I know it's more linear that RPG people like, but repeating the same sort of building three times in a DLC just for the sake of giving the player some choice as to what one they want to do first just doesn't seem like a really good use of that "open-choice" idea.  IMO, Noveria handled that aspect much better than BDtS.  Also, going out to the far corners of the map to locate the engineers really works against the urgency issue of shutting down the torches and I only just recently discovered the grenade cache tucked way over in a corner right near the outer boundary of the map... some exploration is great, but the "incentive" needed to be more in tune with the mission plot, IMO.  One should be more likely to stumble onto the sundry items while looking for the main mission buildings.  In BDtS, the three torchees are clustered near the center of map and so obvious... meaning that you really had no reason to explore the map corners where the other stuff is located.

I've also noticed there tends to be one straggling Batarian behind an automatic door at the back of the building on the bottom floor. It's one of those colony prefab units you see on the overworlds alot but there's one in the building there for some reason. 9 times out of 10 I've cleared all the enemies and Balak won't trigger and I'll go over there and as soon as the door opens, out pops a Batarian and once you kibosh him, Balak shows up.

Tali's recruitment mission is ME2 is one of the better designed maps in that game, with the choice to fight in lower vs upper areas, the solar environmental hazard and the finale with the colussus with multiple areas to attack it from, plus the gameplay affecting choice of whether or not to have Kal'Reegar assist you(and him surviving if had him support you if you were fast enough).

You're right.. I don't know why I forgot it.

It's too bad he doesn't show up in ME3. I had to invent an elaborate story for myself, from the email message. Palaven might've been great to play through.