The boring account that makes every other financial plan survivable. How big should yours be, and when will it be full?
Rent/mortgage, food, utilities, insurance, minimum debt payments, transport — the survival number, not the lifestyle number.
3 months for dual incomes with stable jobs; 6+ for single income, variable income, or a tight job market.
High-yield savings or a money-market fund — it must stay liquid and boring.