On the top of my head, probably BIP39[1]. It's the reason why we can use mnemonic phrase backups(those 12-24 word backups that are always recommended that you store offline) with our SPV[2] wallets like Electrum today.
an important upgrade to be sure, but most people take deterministic wallets for granted. segwit was far more
memorable due to the block size debate reaching a fever pitch w/ the UASF movement. segwit activation and the launch of bitcoin cash was the culmination of several years of bad blood in the community. we've never seen anything like it before or since.