Everyone has come to the same conclusion when it comes to what makes the best coconut oil, the best coconut oil for bulletproof coffee is MCT oil. MCT oil is essentially a refined form of coconut oil. It's extracted by running coconut oil through a centrifuge and only taking the middle part of the coconut oil that's become separated. MCT oil is about 10 x times more powerful then regular coconut oil. 1tbs of MCT oil is equal to about 10tbs of regular coconut oil.
Naturally it goes without saying that you should always go for the best quality of coconut oil that you can get your hands on. There's a ton of information online from people who've done the research so finding a good quality coconut oil shouldn't be too hard to find.
In my research, I've found Now Foods Mct 100% Oil to be the best. It's well priced and it comes from a brand that is trustworthy. Keep in mind that because MCT oil is sooo much stronger then regular coconut oil, you'll want to use less then recommended for the first few weeks so that your body can adjust to digesting it.
Speaking of digestion, a lot of people have difficulty digesting coconut oils because their bodies aren't used to it. If you find that your stomach isn't feeling right after trying bulletproof coffee just use less and less each time until you feel good. Keep in mind that as your body learns to digest higher quantities of coconut oil, you can add more and more. It usually takes about 2 weeks of daily drinking for your body to adjust.