The best coolers are the Long Low Pressure Drop (LPD) stacked plate coolers. They are effective, and very reasonably priced. If you are racing, get the biggest Long LPD cooler that will fit (like the 4590) and mount it securely with the metal brackets they provide. They also have the Tru-Cool Max with even more cooling capacity, but I've never used one of them.
Here are some sources for the coolers:
The Long LPD is also sold under the B&M brand, but there may be some slight differences in packaging and parts that are included.