Alrighty, hope this helps.
ME2:
Stolen Memory (Kasumi DLC) is alright. You get a crew member, a mission and a SMG that is actually good. Bit short and nothing amazing, but also nothing particularly wrong with it. Might not be worth whatever price they're asking for it. The character you get is not hugely fleshed out (think Zaeed), but generally likable. If nothing else, it's nice to have someone optimistic and cheerful among your crew of sad sacks, psychos and broken wrecks.
Overlord is good. Not great, but solid stuff. Reasonable length. Entirely unrelated to the main game, although it will lead to some nice cameos in ME3.
Arrival is pretty shit, in my opinion. Feels like a slapdash effort, like the A Team was busy working on ME3 and the B Team was on holiday, so the let the C Team out of the basement to crank out some DLC. Ridden with plot holes too. And not the easily ignorable "eh, whatver" kind of plot hole, but the big throbbing kind.
Lair of the Shadow Broker is the pick of the bunch. Ties in nicely with elements of both ME2 and ME3. Great pacing. Great mix of action and talky bits. Good character bits. I'd recommend it all round, but if you're a Liara fan to any degree whatsoever then it's a total no-brainer. Buy this one if nothing else.
The rest of the ME2 DLC is incidental and can be safely ignored. Generally just weapons, skins and other trinkets.
ME3:
Extended Cut. I thought it was worth it. Took the ending from a dear-God-what-have-they-done D to a oh-fine-I'll-fucking-take-it C in my eyes. For whatever that's worth. Your mileage may, and very likely will, vary. It's free, by the way.
Omega. Alright. I basically enjoyed it, but it's far from mind-blowing. Pretty combat heavy. If you're being economical then you won't be missing anything amazing by skipping this.
From Ashes. I'd say it's well worth it. You get a mission, a weapon and a character. The character is good and fully integrated into the campaign with proper dialogue scenes, inter-party banter and everything.
Leviathan. Ehhhh. It's not bad exactly, but I can't bring myself to call it good either. Just sorta there. Gives some major background on the main story, but it was clearly written after the whole ME3 ending business and feels like a mix of narrative damage control, post facto justification and elaboration. I'll leave you to draw your own conclusions as to whether that's something you want.
Citadel. Okay, I'm just going to let my inner fanboy off the chain for a mement here. FOR THE LOVE OF ALL THAT IS HOLY, BUY THIS SUCKER. Citadel is some of the finest content Bioware has ever produced. It's 4-5 hours or so of condensed joy. It's a bit silly, and if someone had described it to me ahead of time I'd have been dubious as hell, but after playing it I promptly decided it was the best swan song for the series I could possibly have hoped for. It's hilarious from start to finish, absolutely loaded to the gunwales with self-referential humour and isn't at all afraid to poke fun at itself. You get a lengthly series of missions, tons of dialogue, with every single surviving crew member of every game getting their moment, and tons of amusing banter between said crew members. The only bad thing I can think of to say about it is that it's kinda disconnected from the main game. Also, some people might find the love-letter-to-the-fans aspect a bit cloying. But I for one cannot recommend it enough.
Aaaaand the rest is either free or is just odds-and-ends stuff of no consequence.