I have to say inside the deku tree in retrospect. obviously. but thats looking back on it. the first time i played the game in 2001, i was 5. first video game ever. (therefore first zelda game) I was so pumped that i got the sword and i had no idea what to expect in the deku tree. i got in, it took me a day or 2 to get the slingshot, and then another week to figure out how to jump onto the web from the top floor. after that, you can imagine it took me a while. after a little more than a month, i made it to the boss. and i couldn't beat it to save my life. after a week of getting owned, i asked my friend's nine year old brother to beat it for me. after i got out of the tree, i had no clue what to do and forgot about the game. then a year later, i picked it up and never stopped. i fell in love with everything in Ocarina of Time. I played through the whole game and finally(but sadly) finished it when i was 10. it was awesome. and the second play through i couldn't believe i thought inside the deku tree was hard.
but still, the first time around i will never forget. and i know that inside the deku tree was hard as hell. so instead of voting it as easiest, i voted dodongo's cavern XD!