Mostly been said above, but here goes. For ME2:
- Lair of the Shadow Broker is easily the best of the bunch, if you're going to get anything for ME2 get that. It's awesome.
- Kasumi isn't essential but it's a lot of fun. Overlord isn't essential either but there's a fair amount to it. And you get to spend more time faffing about in the Hammerhead, if that floated your boat (it didn't for me). EDIT: forgot Zaeed - if it's not already included in your copy of the game, it's not essential but it's a little bit of fun. Kasumi is easily the better of the two.
- Arrival seemed at the time like it should be essential from a storytelling point of view... but if you haven't played it they just hand-wave it away in the opening minutes of ME3. Gameplay-wise it's a bit of a grind.
And for ME3:
- Extended Cut should be free, and it's pretty much mandatory. Unless you want to experience first hand the horror we all went through when the game was first released.
- I'm nowhere near as madly in love with Citadel as most other people seem to be, but it does contain some fantastic character moments. Worth it.
- I had From Ashes on day one so I've never played the game without it. I can see how it would still work just fine. But it's definitely worth having.
- Omega is a guddam mess. Unless you really, really liked the Aria and Omega stuff from ME2 (it's my least-favourite part of that game, FWIW) you can pretty safely skip it.
- I liked Leviathan a lot, but it's not essential.
Hope this helps!