I actually misspoke:
Apparently azúcar can be either masculine or feminine depending on the country. I personally have only heard it as feminine.
The most strength in the word is indeed in the "ú", but I thought that the rule was whether the "a" was strong or not (not necessarily the strongest). It turns out the rule is: You use "El" for words that start with an "a" that is in the strongest syllable. HOWEVER, "azúcar" is "el azúcar" because it is a remnant of medieval Spanish (and an exception to the rule of when to use "el").