It depends on the temp drop in each direction but in general counterflow wins out every time. You will get the highest heat transfer. Flow rate is not that important for watts transferred as it is either going to be a high flow at low temp rise or vise versa. That said, the flow rate is determined by the desired output temp (and supply temp). If you need 35C, go slow and save fan/pump power but if you want 25C you may need to spend a bit more energy to do it.
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