ME1 DLC
- Bring Down The Sky: it's given away for free now, it's a bonus stand-alone mission that doesn't have anything to do with Saren and the main plot.
- Pinnacle Station: give it a miss. very boring training camp - and you get a much better version in ME3.
ME2 DLC
- Zaeed "The Price of Revenge": your receive Zaeed Massani as a bonus squadmate, and he's really a beast! one of the best characters in the trilogy. such a shame his voice actor is now dead.
- Kasumi "Stolen Memory": you receive Kasumi Goto as a bonus squadmate, and she's a sweetie
and pretty useful I might add. she's also got a very cool loyalty mission.
- Crash Site: it's not an action mission, you just land on a planet to retrieve some things that hold great sentimental value to Shepard & others.
- Project Overlord: it's actually a pretty decent set of stand-alone missions, so it gets my vote!
- Lair of the Shadow Broker: definitely a must-have, especially if you're a Liara fan. even if you're not, it's got great action and answers a few questions about Shepard.
- Arrival: another must-have as it bridges the ending of ME2 with the beginning of ME3. it's more action-oriented and it's a solo mission, without squadmates. It will remind you of "Bring Down The Sky" as both DLCs are situated on asteroid surfaces.
ME3 DLC
- Extended Cut: it's free and it's a must-have. SERIOUSLY.
- From Ashes: oh definitely a must-have! You receive Javik as a bonus squadmate, who is the biggest, funniest troll in the ME trilogy.
- Leviathan: it's lore-heavy and delves deep into the origins of the Reapers. the action is ok, but for me it could've been better. If you prefer lore over action, you'll love it.
- Omega: for me it's a must-have, especially if you end up loving Omega itself in ME2. it's action-heavy, not big on lore. lore-lovers hate it, action-lovers love it. personally I love it. you can't bring existing squadmates with you, but you get to team up with a real badass Asari and a female Turian mercenary. they are Omega-exclusive squadmates so after the mission is over, you can no longer take them with you.
- Citadel DLC: it's a stand-alone mission that has nothing to do with the main plot. on its own, it's a fun side mission, and you get to host a huge party with all the squadmates you teamed up with in the trilogy. it also has a battle arena and games. it's the final DLC of the trilogy, and personally I wish they had spent all the resources to improve the final ME3 mission.
personally whether I like or dislike certain DLC, I got all of them because they're chunks of the games. without them it's like playing only 60% of each game.