Best retort to "send your best offer"?

[Ben Ballinger]

Seems like every time I put an offer on a property that is less than asking price (or even when it is asking price), they come back with "send your best offer". I get it from their perspective, it's a smart response, but what's the best way to respond to this? If I just give them the best offer then it completely eliminates any negotiating ability, so is there a way to respond that opens up the discussion for negotiating back-and-forth?