"Always Be My Baby," "Fantasy," and of course, "All I Want For Christmas Is You." I do think "We Belong Together" could end up being a signature hit, but it hasn't received as much recurrent airplay as I expected it to.
"Always Be My Baby" has always been one of my favorite Mariah songs of all time, but I didn't start hearing it as a recurrent on the radio as much until the Emancipation era -- and since then, I've heard it regularly on so many different stations in different states each year. People remember "Hero" for sure, but I think in the United States, after "All I Want," "Always" seems to me like her signature song.