varies a bit by group size. Consumer Reports tested and provided a report in their magazine about two months ago (Sep or Oct 08??). For group size 35 and 65, the two I am most concerned with, Walmart Maxx came out well.
For selected sizes, Kirkland Costco came out well, but not on all sizes, particularly my two. Since I travel places where there are no convenient Costcos, I hesitate to buy their batteries and get stuck away from their stores, so for me, Walmart is a more logical, less risk choice.