US Radiator offers a few different core options in OEM configurations. Can't tell you if they're the "best" or not, but I went with one of their radiators for the Duster we're building. A bit pricey, but the quality looks to be there...
Here's a cool video on how they're built:
http://www.usradiator.com/news/usradiator-factory-tour/
(and no...I don't work for them, just went through this same search about 6 months ago)
Tom