Segregated Witness (BIP by Pieter Wuile, Eric Lombrozo, and Johnson Lau) and Taproot (BIPs by Pieter Wuille, Jonas Nick, Tim Ruffing, and Anthony Towns) are the two largest changes ever made to the Bitcoin protocol. 

The former fundamentally changed the structure of Bitcoin transactions, and in the process Bitcoin blocks, to address inherent limitations of the previous transaction structure. The latter rearchitectured some aspects of Bitcoin’s scripting language, how complex scripts are structured and validated, and introduced a new scheme for creating cryptographic signatures. 

Those are both massive changes in comparison to say, adding a single opcode like…

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