The honest answer to "what's the best X" will always be "depends what your deck is doing" - for example, Mass Commercialisation is a 0-cost Operation that often gets me 10c or more in my BoN deck, which is phenomenal value, but only because of the deck I'm playing.
That said, there's one econ card (Violet Level Clearance) that's good enough it's currently banned (mainly because there's an ID that abuses the shit out of it), so that'd be one way to answer.
Fucking around aside, some good econ cards are:
Marilyn Campaign - a decent econ asset in its own right, but what it has over every other econ asset is that it recurs itself, so you get to have that drip econ again and again and again. Drip econ in general is good because you're not spending clicks to make money, but often it'll run out. Less so in this case.
Breaker Bay Grid - not an econ card in and of itself, this region makes all assets and upgrades you put in the server 5c cheaper. You want to know how many assets and upgrades cost more than 5c? Exactly one. And it costs 6c. So this makes any asset econ free.
Ultraviolet Level Clearance - it's awkward to use, but it's a big econ boost. It can be wasteful - it eats up a whole turn and might well cause you to discard if you're near max hand size. However it gets a lot better in conjunction with:
Bryan (Barney) Stinson - like BBG, not technically an econ card, but an econ card enabler. He lets you Remove From Game any Transaction Operation from archives - no matter how it got there - to play it ignoring all costs. Your basic Hedge Fund goes from a 4c gain to a 9c gain, and things just get better from there. Your opponent does have to be poor for him to work though - protip: pack tools to make them poor
Celebrity Gift - this is a pretty good econ card anyway, having a fairly good gain for a low cost, but its the other bit of this card that makes it notable - revealing a hand full of deadly traps and/or agendas is great mind games.
Cont.