Lair of the Shadow Broker for ME2. Don't buy Arrival as it's bad. Overlord if you're buying a second DLC for ME2.
For ME3 it really depends. If you want lore-heavy stuff, Leviathan and From Ashes. Javik is one of the best characters in the trilogy, so I'd actually recommend From Ashes over Leviathan, as a lot of Leviathan is just fleshing out what the Extended Cut Catalyst convo says.
Citadel is the best bang for your buck though. A ton of character stuff, along with a light-hearted plot and a surprising amount of combat through the Armax Arena. It's really just the main plot and after-party in Citadel that are joke-y, the individual character scenes the DLC adds are stellar.
If combat is REALLY your thing, then get Omega, but if you're strapped for cash then my recommendations are Shadow Broker, From Ashes, Citadel, in some order.