What is the optimal way to stress test a server?
In terms of just sucking up a lot of memory, the worst thing I've ever done is crowd a few chunks with obscene amounts of items.
If you create a bunch of Igneous Extruders and set them to create cobblestone, and pipe it out of the machine to fall on the floor, they'll just pile the material up indefinitely until it lags your server to the point of being unusable.
Personal experience. (This is why you build redundancies into your automatic warehouses children.)
Use a very large amount of IC2 solar panels. They are poorly optimized and perform a check of every single block above them (as there is no way to do raycast in minecraft that is known). A carpet of solar panels at bedrock will really put a strain on server CPU and memory.