Chase Freedom is 5x at Costco for the first $1500 of Costco spend until the end of the year (effectively a minimum of 7.5x if you pair with a CSR or 6.25 with a CSP/Ink, potentially more if you transfer points instead of using Chase's travel portal). After December 31st, Freedom Unlimited would net you a minimum of 1.5*1.5 = 2.25x paired with the CSR/1.875x with the CSP.

This all assumes you have use for UR points, of course. If you prefer cash back, Fidelity's Visa card (2x on everything) or even the Citi Costco card might be better suited for Costco spend after the end of this year.