The crypto industry boasts diverse approaches to smart contract execution and decentralized applications (DApps), driven by the imperatives of scalability, security, and efficiency. These innovations enable developers to create increasingly sophisticated applications. However, what distinguishes smart contracts across different blockchains, and which platform reigns as the smartest?

Turing completeness, a fundamental aspect of smart contracts, refers to a system's capability to perform any computation given sufficient resources. Named after Alan Turing, this concept underscores a blockchain's ability to execute complex operations. Leading platforms like Ethereum, Internet Computer (ICP), Polkadot, Cardano, and Solana each employ distinct strategies in harnessing Turing completeness and deploying smart contracts. This analysis delves into their approaches, elucidating their specific attributes and contributions to the decentralized ecosystem.

Ethereum Smart Contracts: Ethereum's hallmark is the Ethereum Virtual Machine (EVM), facilitating smart contract execution and DApp deployment. Turing completeness empowers Ethereum to support intricate smart contracts, albeit requiring a gas mechanism to manage computational resources. Despite notable security challenges, Ethereum's ecosystem thrives, offering a plethora of DApps and services.

Internet Computer Protocol Smart Contracts & Canisters: The Internet Computer Protocol (ICP) introduces canister smart contracts, combining code and state for versatile computation. A novel reverse gas model streamlines transactions, while robust security and interoperability features enhance scalability and efficiency.

Polkadot Smart Contracts on Parachains: Polkadot's architecture enables interoperability among parachains, each tailored for specific use cases. Supporting multiple smart contract languages, including ink! and EVM compatibility, Polkadot fosters a diverse ecosystem of DApps and services.

Cardano Smart Contracts: Cardano adopts a dual-language approach, featuring Plutus for Turing-complete contracts and Marlowe for financial agreements. Security-centric design and formal verification techniques ensure robustness, with an emphasis on scalability and usability.

Solana Smart Contracts: Solana prioritizes speed and scalability, leveraging the Solana Virtual Machine (SVM) for high-throughput smart contract execution. Rust and C support efficient contract development, while Solana's stateless architecture enhances security and scalability.

In conclusion, the blockchain ecosystem's array of approaches to smart contract execution reflects its vibrancy and dynamism. Each platform offers unique strengths, from Ethereum's expansive DApp ecosystem to Solana's unmatched speed. Ultimately, network effects and adoption will delineate the prowess of each chain, heralding a multichain future in the global economy.