I like iancoleman one. With dice to generate the randomness. Use it in a offline computer booting a fresh linux installation or tails os. Can't be colder than that. 

quark

OK, I got sent some nice tools and guides on Twitter. I'll add them here for anyone else who is interested:

bitcoin

How do I derive Bitcoin addresses from a seed (12 words) OFFLINE?

flix2

OK, I got sent some nice tools and guides on Twitter. I'll add them here for anyone else who is interested:

https://github.com/axic/bip32-path

https://estudiobitcoin.com/do-you-trust-your-seed-dont-generate-it-yourself/

https://mdrza.medium.com/how-to-convert-mnemonic-12-word-to-private-key-address-wallet-bitcoin-and-ethereum-81aa9ca91a57

https://github.com/iancoleman/bip39