IMHO the Borg.
When you fight the Dominion, they expend energy-ships-manpower. Attrition against them works as seen in DS9, especially when the supply line wormhole was cut.
When you fight the Borg... if they pick their targets well... they get stronger. More drones. More industrial capacity. More capability. If they wanted to overrun the galaxy they could, one world at a time.
I think Voyager made it pretty clear that the Borg didn't sweep the Delta quadrant not because they could not, but because there was little they didn't already have. The Federation, 200 years before our heroes debut, would be the same. What possible use is 2012 earth to a galaxy spanning group like the Borg?
So down to the one cube question. I would argue the first "one cube" visit is a direct result of the Q Who encounter, where the Enterprise suddenly vanishes while under attack. The Borg would think that's really cool technology to have and that same cube could be tasked to check on that. In that context, its no accident they're near Picard and particuarly treat him specially. Unlike billions others.
The First Contact "one cube" argument is harder and there's no good reason if you believe they were there to conquer the Federation. Of course, you'd go back 100 years first... or at least go back 100 years in a way that the enemy's warship could not follow you!
So my personal belief: the Borg are employing strategy, not war. They want something from the Federation for the price of the "one cube" which is not that high given what we know. They might want the Federation's R&D capability...proven very strong... or my view, they might want to toughen up the Federation for a threat that the Borg know of that is coming. Why not position the Federation to put up a tough fight against an entity that the Borg are concerned with? Species 8472 is an example, but i'd lean to something on the far side of Fed Space (relative to where the Borg are based in the Delta quadrant) that the Borg are concerned about.
Strategy, not a bad attempt at conquest. Shame we killed them off in the books.