The best coffee for bulletproof coffee (as far as we know) is Upgraded Coffee, sold by Dave Asprey. Dave Asprey through obsessive trial and error, found that the coffee must be rinsed in clean water soon after it's been picked to prevent mycotoxins (mold)(toxic to the human body) from taking hold. They're then sent to the no# roaster in the USA (roasting enhances antioxidant capacity).
If the Upgraded Coffee sold by Dave Asprey is out of your budget, be sure to choose a high quality coffee that's pesticide/herbicide free and organically grown. High quality coffees are definitely more expensive then low quality Starbucks coffee but if you're going to be drinking it everyday, it should be the best you can afford.