Depends on your definition of "spicy". Are you talking about foods that contain a lot of spices, or food that is "chili hot". For the former description, my guess would be Indian. Indian food can contain a myriad of spices resulting in complex and "layered" tastes. No other food comes close, in my opinion.
Now, if you're talking about "chili hot", then there are some varieties of Thai peppers that can very nearly kill you! Ok, I overstate that! I frequent a Thai restaurant in my neighborhood, and I ask for something called "chili oil" as an accompaniment. That stuff makes my scalp sweat, and gets my vote for the "hottest" food!