Any licensed Dexron VI will be fine, so just shop on price
I like Castrol's Full Synthetic ATF. It often goes on sale a few times a year at the parts stores for $5/qt. Castrol Full Synthetic is licensed Dexron VI
Maxlife ATF is also very popular on here, even though it is not licensed. Widely available at Walmart at a great price. The best choice if you're out of warranty.