I put mine as far away from the point where the water reenters as I could. logic just seemed to suggest this would be the best way to make sure the whole pond was cycled, and not leave 'dead spots'.
If that is also the deepest spot, a good idae is to put the pump up on something, off of the the bottom. In the event the worst happens - a band of raccoons amuse themselves with your plumbing eg - the pump will not completely drain your pond, as there will always be water below the level of the pump.
Cheers,
Mike