Hmm. Difficult question. I kind of want to say OoT, or maybe TP, but...
I think I'd go with Spirit Tracks. That was the first Zelda game -- actually, the first video game ever -- that I waited for, preordered, followed information on, counted down the days until its release, and tried to get on the first day, even though Game Stop was mean and didn't get it in until the day after. XD
I started my Zelda blog and called myself a Zelda fan after playing OoT, though, which was my fourth game. But I beat that before TP, so... oh, I have no idea. XD I mean, after PH (my second game, following TWW) was the first time I actually started caring that these games had the title of "Zelda" in it, so maybe you could even argue there.
*shrug*