Which resource do you find to be most valuable? Which resource have you noticed caused the humans to lose, or got closest to it. My thoughts:
Population: Seems to be relatively easy to hold onto, given you never get overwhelmed by raiders. We've had some pretty overwhelming situations, but the way turns go it seems to take some pretty bad luck. Although we've always had a piolet. I imagine if all the pilots become cylons this could be hard for the humans to hold onto.
Morale: Seems to be the most volatile. Lots of cards to increase and decrease this resource. Hasn't been a tipping point in any of our games.
Food: This, in my experience, is the resource that sinks fleets. Food shortages are common and when pulled insure at least one food gets reduced. This is very important when the end of the game is near and all human resources are almost depleted. If food is ever reduced to one, then the game can end easily by pulling a food shortage.
Fuel: This starts the lowest of any resource and will almost always be reduced when jumping. It's balanced such that, on average, about 8 fuel will be needed for your jumps alone. This means that small increases or decreases in fuel can have large complications later in the game.