Systems with a security level of 0.5 or above will have barely any NPCs in the asteroid belts and almost all of them will be frigates with tiny bounties. Low security systems with a security rating of 0.4 to 0.1 are significantly better. The asteroid belts there spawn packs of cruisers, battle cruisers and occasionally a battleship. Unfortunately, this is not really enough to outweigh the risks of encountering roaming pirates. It is a lot safer to run missions in high security space than to venture into low security systems, and level 4 missions will make considerably more eve isk per hour.
By comparison, the lower population density of most nulls regions makes them much safer and the NPCs here are for farming. In the relative isolation of nulls, you can often go ratting for hours without encountering another player. All nulls systems can get spawns containing battleships, with bounties ranging from 500k to 2.05 million ISK per ship. Some star systems will tend to get higher bounty battleship NPCs and more triple battleship spawns than others, with a higher chance of getting valuable faction commanders.
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