Ethernaut
Ethernaut is a web3/Solidity based wargame played in the Ethereum Virtual Machine. Each level is a smart contract that needs to be 'hacked'.
Capture The Flag Challenges
Coding Challenges
LeetCode
One of the leading platform for coding skills. It offers a plethora of algorithmic challenges that range from data structures to databases.
Smart Contract Engineer
The LeetCode for Solidity engineers. Look for The Coding Canuck in the Top 100 leaderboard.