It depends which kind of decentralization you mean — here’s how the leaders actually compare.
If you mean consensus distribution, Cardano and Polkadot lead by Nakamoto coefficient. If you mean node count and credible neutrality, Bitcoin is unmatched. If you mean a healthy mix at scale, Ethereum is the strongest smart-contract chain. There’s no single winner — only the best fit per dimension.
The most decentralized smart-contract chain by validator breadth and client diversity.
For consensus distribution, Cardano/Polkadot; for credible neutrality, Bitcoin; for a decentralized smart-contract platform, Ethereum.