I will directly answer to this question without quoting OP or someone else's posts.
The biggest difference between Bitcoin and altcoins is that Bitcoin is the truly decentralized currency with the highest adoption rate (don't forget the highest market capitalization too). And the reason behind these high numbers is that it's truly decentralized. Who created Bitcoin? Who owns Bitcoin? We don't know man since the day one. Is there any altcoin in such condition? I don't remember any. That's why businesses and other organizations aren't afraid to invest into Bitcoin. Basically, an unknown person created it, dropped it and that's it while that can't be said on other altcoins.
It's true to say Bitcoin blockchain is a truly decentralized blockchain, bitcoin is a native currency on that decentralized blockchain. bitcoin itself can not be decentralized as a native currency if Bitcoin blockchain is not decentralized.
Some biggest differences between Bitcoin and altcoins.
First of all, quantity: there is only one Bitcoin blockchain but there are many altcoin blockchains; there is only bitcoin currency while altcoins have thousands of altcoins and tokens.
Secondly and very importantly, since 2009, Bitcoin has never died, rolled back, did not have any 51% attacks while there are altcoin blockchains with 51% attacks, blockchain rollback like Ethereum after the DAO hack, and a lot of dead altcoin projects from coins to tokens.