What is the optimal approach to hacking?

You should have high virus strength and coherence, you should fly a ship that has bonuses to these stats.

There is no surefire pattern to the nodes, they change depending on the difficulty of the site, and this only effects the density of the spawns.

The one constant is that if there is a bad node, and it is surrounded on all sides by nodes, the system core has to be one of those nodes.

The optimal strategy is to try and find this bad node by sticking to the center of all the nodes, only revealing the nodes that are surrounded on all sides by other nodes.

By following this rule you either find the bad node, and thus the system core, or you find a positive node or nothing at all. There is a fairly high probability that the center nodes don't have a bad node, in this case the system core could be anywhere, but usually spawns near a bad node.

I've had the best luck clearing all the center nodes I can, knowing that they have no bad nodes, then I explore, and destroy any bad nodes I come across, always destroy the buff nodes first, as the system core has a high probability of being behind a bad node.