Well, you havn't said what type you need so here's what I would use if it were my tank;
My first choice would be an Iwaki external with a Jap built motor. THe model I would use is the MD70RLXT assuming I was only needing a free flow return back to the aquarium. Iwaki rates this pump at 1500gph @ 4ft head. Put a ball valve on the outlet side of the pump and adjust it as needed.
If I was forced to use a submersible, I would choose the Mag-Drive model # 18 from Supreme. It is rated at 1200GPH @ 5 ft head.
I don't use RIO anymore, a few years ago it seemed the quality of thier pumps went way down hill. I was having short out problems with them regularly. Ironicly, the RIO pumps I have from before that time are still working fine. I am guessing there was a change in materials or manufacturing but I don't know for sure. I know someone mentioned little giant, I don't care for thier pumps. THey are kind-of noisy and seem to have a short live compaired to Iwaki. I have a box full of old little giant pumps I am not using any more.
Hope this helps.....
Joel