Itself.
Ethereum's biggest competitor is Ethereum itself. I mean look at Ethereum's gas fees. I also joined an airdrop a while ago where I need to bridge from Ethereum Mainnet to that project. There is no problem when I bridge from ETH to the project, but when I want to bridge it again from the project back to ETH mainnet, the transaction fee is a whopping $67. To bridge $50, I need to pay $67 in fees. Who in the hell will do that?
With the Dencun upgrade coming a few weeks from now, it focuses on security, scalability, and reducing its gas fees. I hope that this will be the solution to the ridiculous high transaction fees. Surprisingly, there are still new projects that are being created under Ethereum Blockchain. Well, as for other competitors (projects), I look at Polygon, Arbitrum, and Optimism.