Kewpie Mayonnaise
http://www.amazon.com/Uwajimaya-Kewpie-Mayonnaise-17-64-Oz/dp/B00023T3IA/ref=cm_cr_pr_product_top
I purchase this mayonnaise regularly, usually more than one bottle at a time. I use it for making Japanese dynamite sauce, although it is very nice in sandwiches as well. This mayonnaise has a flat taste to it, and is not at all sweetened like American-style mayonnaise.
The product is tops but the packaging is a little odd. The mayonnaise itself comes in a squeeze bottle, and the squeeze bottle is packaged in the red-printed cellophane bag you see in the picture. Well, when you remove the bottle from the cellophane bag, you are left with a plain bottle, with no markings on it whatsoever. You will know what is in the squeeze bottle, but others won't, so you just need to communicate that to anybody that has access to your refrigerator.
A nice little bonus is that the cap on the squeeze bottle has a rosette tip, so you can make fancy little mayonnaise decorations straight out of the bottle if the occasion arises.