Essential tokens for Ethereum ecosystem

If I want to invest in a token that is absolutely vital to Ethereum's infrastructure and integration, but isn't ETH, which one stands out as the most essential?

I am looking for a token that is critical to the Ethereum infrastructure and deeply integrated into its ecosystem, excluding ETH itself. Which one would you consider the most essential?

I want to invest in a token that is deeply integrated into Ethereum's core operations. Aside from ETH, what is the single most vital token to own?

Besides ETH itself, what's the single best token that is most deeply integrated and essential to the Ethereum ecosystem?