1. Lair of the Shadow Broker, Mass Effect 2.
Great pacing, great music, some great set pieces. Also, the ability to respec your squadmates (finally!) plus a bunch of upgrades.
2. Overlord, Mass Effect 2.
Fantastic music, great atmosphere. Prometheus station--the crashed Geth ship--is my favorite sequence in the entire DLC and possibly the entire trilogy.
3. Bring Down the Sky, Mass Effect 1.
Some good fights, and a great difficult choice to make at the end. Also, Batarians.
Trying to decide between the rest of the DLC is difficult as they all have good and bad points that make them even out. Probably:
4. Stolen Memory (Kasumi DLC), Mass Effect 2.
New squadmate, but she (Kasumi) is the only squadmate available for this mission. Mostly a straightforward fight down narrow hallways. The last fight is fun, but it can be buggy at times. New upgrade (Tech Damage) and new weapon in the Locust SMG which benefits most characters.
5. From Ashes DLC, Mass Effect 3.
New squadmate (Javik) with a lot of interesting content and dialogue, new weapon, some good fights. New upgrades for Intel terminal.
As for the other DLC, I will not rank them, but I will explain what I think of them:
- Omega DLC, Mass Effect 3. I had a lot of fun with this DLC, but I have only played it once so I cannot form a good opinion on it yet. The last fight was the toughest because I specced my Soldier "wrong" (I picked the "wrong" evolutions of Adrenaline Rush, specced into Grenades). The rest was incredibly easy because it is Mass Effect 2 combat. I loved the varied locations. Aria was a decent squadmate, but ultimately worse in combat than everyone else on my squad except James. I disliked how Nyreen kept joining and leaving the squad.
- Leviathan DLC, Mass Effect 3.
Also a lot of fun. I would rank possibly rank this higher but I have only played it 2 or 3 times. Great set pieces, but ME3 combat is an ultimate letdown. Lots of new mods and some new weapons (Raider, Argus) that I have not used, and a new power (Dominate) which I have used but think is unnecessary. Some good non-combat/investigative sections that seem to drag out too long sometimes. Scanning for the new war assets can take a long time.
- Arrival, Mass Effect 2.
Fun, but fighting by yourself can be a pain at times. The Battle for Object Rho can be difficult but fun, but also incredibly buggy a times (especially as an Infiltrator). Dr. Kenson is the worst squadmate in the entire series, good thing it is only temporary. Some new upgrades but nothing that makes a big difference.
- Price of Revenge (Zaeed DLC), Mass Effect 2
Some decent fights and an interesting choice that also leads to different upgrades.
- Pinnacle Station, Mass Effect 1
Can be fun at times, but ultimately boring. At least one scenario can be extremely difficult to pass on Insanity.
As for the weapons packs in ME1 and ME2, overpowered weapons are overpowered.