Due to the lack of details, it is difficult to make a recomendation, but some thoughts.

Both as an AGI challenge and for general human safety, business intelligence datawarehouses are probably a good bet. Any pattern undetected by humans detected by an AI could mean good money, which could feedback into more resources for the AI. Also, the ability of corporations to harm others doesn't increase significantly with a better business intelligence tool.

Virtual worlds - If the AI is tested in an isolated virtual world, that will be better for us. Test it in a virtual world that is completely unlike ours, a gas giant simulation maybe. Even if it develops extremely capable technology to deal with the gas giant environment within the simulation, it would mean very little in the real world except as a demonstration of intelligence.