The Triforce would be the symbol I would say that defines Zelda.
However, I believe the red "Zelda" in "The Legend of Zelda" would be the logo that defines the series. I just did a quick Google image search[/URL], and saw the red "Zelda" before the Triforce.
I would probably agree with most of the people posting in this thread and say that the hylian crest (bird with triforce above it) is the symbol I would use.
Another suggestion I would say is the Gate of Time from Skyward Sword. There is a lot of symbolism in it. There is the triforce in the middle and it kind of represents that the series is spread over time.
Many other things as well could act as the symbol of Zelda.