I would say MCT/Coconut oil. If you eat fish 2x-3x/week you'll get sufficient benefit from the omega's.

Here's some solid info on MCT/Coconut oil:
https://authoritynutrition.com/mct-oil-101/
***key takeaways = C8 is the best fuel, C10 is next best. Try to steer clear of C12 for fuel - but it does have anti-viral, anti-microbial, & anti-bacterial potential benefits.

If you're attempting to get into Ketosis, you can also supplement with exogenous ketones - if your hunger feels uncontrollable. Exogenous ketones are a hot topic of debate for fat burning purposes:
https://www.t-nation.com/diet-fat-loss/avoid-this-ketogenic-rip-off

MCT's (rather than MCT oil per se) all the way - pure mct oil, or from coconut oil or grass fed butter
my personal pref is coconut oil as its nice and is food!

in terms of most ppls body's ability to consume, they are preferable in reverse order (to prevent diarrhea etc.). But in terms of ketone production, (which I think is what you mean by fat burning) they are most efficient in the order listed - but of course, Ketone salts will be with us properly soon and can jump to the head of the list then

In terms of disease such as dementia (alz park etc.) the same applies, but you can keep carbs in the diet as the ketones will cross the BBB and fuel the brain regardless

In terms of cancer; you would need to remove all carbs (bar highly fibrous carbs like broccoli) and then just go keto as a lifestyle and that may very well completely starve the cancer of its fuel. Keto is especially good here, as it limits both carbs/sugar/glucose foods and foods that convert to glucose during digestion to cut off the fuel

In terms of diabetes, same again but you need to go keto to kill the diabetes

In terms of endurance sports - Ketone salts or MCT oils as the fuel source

I'm assuming you mean coconut oil by mct oil? Why not use both? I always( no matter cut, bulk, maintenance) supplement with fish oil but I also eat about 2 pounds of fresh salmon throughout the week.