Thick savory Japanese rice crackers (senbei) like these. I can get them at Mitsuwa, but don't get there often enough.

Like others have said, potato chip and tortilla chip flavors you can't find in the US -- sunday roast, roast chicken, shrimp & mayo, etc.

Cadbury's Almond & Apricot Bar (which I think has been discontinued) in the UK, as well as Fudge Bars, Star Bars and Candy Shrimps. Wacky Japanese Kit Kats.

In the pre-Trader Joe's days, Dutch stroopwaffels! I cannot tell you how many of my college friends got hooked on them thanks to me. And now that I can buy them in any number of places in the US, I probably haven't eaten a single one in the last 5+ years.