As far as best
and easiest, this would be the one:
B&M 70268 - B&M SuperCooler Oil Coolers - Overview - SummitRacing.com
I say this because it has the 3/8" barbed inlet and outlet ports which are easier to install that some of the other coolers with threaded inlets and outlets that require additional fittings. Furthermore, it's the largest one they carry with these fittings and is also a bar and plate design vs. the tube and fin. Bar and plate is more efficient and much more durable.
If you want just a little extra work, you can go with this bad mammer-jammer. I have it and it's the biggest external cooler I could find. It has 1/2" female inlet and outlet ports which requires extra fittings and so forth but it's epic big:
B&M 70274 - B&M SuperCooler Oil Coolers - Overview - SummitRacing.com