Best sources are from a mix of plant (flax and chia) and animal (krill and cold water fish). Plant sources still have to be processed in our body to a form (EPA and DHA) to be effective. Unfortunately our conversion rate is very low, around 5% and it needs lots of coenzymes (vitamins). if you are missing the vitamins then it cant be converted.
There is a growing body of evidence linking DHA to cognitive function and various studies have reported that regular consumption of omega-3-rich food could prevent age-related cognitive decline. The studies consistently show a dose-dependent inverse association between dementia and fish consumption. That is, the more omega 3 you consume the lower the risk.
On the other hand, meat consumption was found to increase dementia risk .
Alzheimer's disease is the most common form of dementia and currently affects over 13 million people worldwide. This number is increasing and we know how to reduce it. Increase your omega 3, lower your meat and sugar.