A car heatercore is the most cost efficient radiator we can get.
That aside, if you look at Hayden tranny (car transmission oil) coolers, you'll find that although they look like a simple tube with fins, they have turbulators built in, which decreases the heat resistance (and increases the pressure drop). I believe that that solution is actually one of, if not the most efficient design.
The copper tube idea can't match either of the above, because they do not induce any turbulence in the water flow.
That aside, you also have to try to maximize the surface area, for the air flow. A heatercore does this extremely well. A round tube provides the least amount of surface.
If I was going to try to make my own, I'd use square tubing, and run a chain of turbulators through it. Ideally, there would be aluminium fins, press fitted onto the tube (how? I don't know).
Good luck.
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