Here is the correct answer if you believe in math:
All of them
Never put all your eggs into 1 basket is more than just an ancient parable, but a math equation based on the probablity of every one of your eggs getting simultaneously (in this lifetime) stolen.
Are you implying splitting up funds into different wallets, secured in different ways?
It seems to me that, mathematically speaking, if the probabilities of funds getting stolen of the different methods is not equal, then, compared to just using the most optimal method for all, you are actually increasing the chance of some of your funds being stolen. I guess the problem is that we don't necessarily know what the probabilities of theft are for each method.
Something else to consider with brain keys- you basically always have your funds "on you", which means they can be revoked from you via violent force (i.e. torture). Therefore you not only have to keep the brain key secret, you also have to keep secret that you have a brain key at all. Maybe have two brain keys so you can give one away as a decoy.
Another thing- you should know what is the decryption method needed to obtain your private key. 20 years from now, software and wallets can change, and wallet generation methods may change as well. If you know though, that you just need to take the SHA256 of your 16-phrase mnemonic (for example), then as long as you have access to a computer you can obtain your private key.