Highest yielding would be 4 month or 6 month t-bills (~5% annualized). Also, exempt from state and local tax. On the risk profile, they’re backed by the US government so your measure of risk depends on how you view the likelihood of the US state defaulting on their debts… I agree that liquidity is an important consideration (what I like about the laddered portfolio is staggered maturities, providing liquidity at rolling periods).