In 2008, following the bankruptcy of Lehman Brothers, the venerable Reserve Primary Fund broke the buck: It held millions of Lehman's debt obligations, and panicked redemptions by its investors caused its NAV to fall to 97 cents per share. The pullout of money caused the Reserve Primary Fund to close and triggered mayhem throughout the money markets.
Looking closer we see that if you can meet the $3,000 minimum investment size, Vanguard’s VMMXX offers an appealing combination of relatively high returns with low expenses. If you don’t have a large amount of cash to invest, you should note that both FMPXX and FIDXX have $1MM investment minimums. If you have less than $1,000, SPRXX doesn’t meet the yield and expense ratio thresholds we laid out, but it doesn’t have an investment minimum.