Hello Shadow,
I thought heat rises, so the coldest parts should be the lowest...
Yes heat rises and cold sinks.
The cooling coils are in the freezer (second link is a nice picture:)
To cool the refrigerator, its heat needs to go up and the freezer coolness goes down. By regulating that relationship the proper temp can be kept. With freezer in the bottom need a fan to make it work.
The bottom freezers have pull out drawers, this mechanism is complex adds to cost, may offer places for heat loss or things to impact efficiency. Side by sides have large suffice areas for the freezer, thus efficiency suffers. Top is the place of choice, most development work has been on the top, so this is where I would expect the best.
Diagrams of various fridges:
http://www.repairclinic.com/0088_4_1.asp
Top Freezer: (from first link)
http://www.repairclinic.com/0100_2.asp
Good advice on:
http://michaelbluejay.com/electricity/refrigerators.html#energystar
My Fridge is like link 2, except heat discharges in rear. Whole back of Fridge gets warm, put a temp probe on top. 80'F. Have a cold air return in the wall behind Fridge. Cut a hole and blocked the air from coming up the back. Top now reads 70'F in 74'F room. I got my Kill-o-watt meter after I did this so have no idea how much energy it saved, math suggests 22%. Looks like most of the new fridges discharge the heat to the front, much better than to back.
The efficiency is greatly effected by two things. The compressor and insulation.
You can buy a nice Fridge and add more insulation. 4 x 8 sheet is like $12 (not blue or pink stuff, want the silver / black stuff). I measured my fridges insulation at 1.125" for refrigerator and 1.625" freezer, By doubling the thickness I would expect the cost to drop in half. Unfortunately I do not have room to do this. If I was willing to tear up my walls and cabinets then I could, not worth the cost. Make sure if you try this you understand how fridge is constructed and don't cover things you should not or it will not work, could even break it. Manual frost fridges are more delicately balanced, may not get the benefits or cause an imbalance between ref and freezer.
Looking over the super efficient units, their only claim to fame is the thicker insulation. Compressor does not seem to be any better. Yet cost 2-3x more than a normal unit, which is frost free!
I checked this by using a spread sheet, measured all the areas and expected heat loss, can match their numbers with thicker insulation for much less dollars. Might not look as good unless you can build in the unit! Built in units are offered today at huge cost adder, you don't have to pay their $ by doing it yourself.
I was very surprised the energy cost of the refrigerator compartment is almost the same as the freezer. Area is the difference. Hope this was helpful.
Have fun,
Scott.