My worst was entering Canada at YYZ in June of 2008. I was beginning a 2 week vacation by spending a few days with a friend before heading on to Korea and Japan. Once I said I was there to visit a friend, I was grilled about how long I was staying, where I was staying, what I did for a living, how much I made, how long I worked there, wanting to see my onward ticket, etc.
I mean geez, does Canada have that big of a problem with Amercian overstays or illegal immigration to warrant that?
Ironically, at YUL where I expected more hassle because I didn't speak French, it was quick and painless.
EU's always been easy. Japan and Korea are annoying due to fingerprinting and photos, but other than that, have been painless. In those countries, if you're on certain prescription medicines that are legal here but banned there, you can be in a world of hurt without proper clearance prior to entering the country.