Obviously, Inside the Deku Tree, because it was meant as a easy dungeon to give you the feel of what the other dungeons ahead are like to give you the purpose of each dungeon. Map, Compass,Sub-Boss, Item, Boss. Then they get a little harder after that and harder after that until you reach the final dungeon you probably already have the feel to it.
Inside the Deku Tree is by far the easiest dungeon in OoT. There are several rooms that are entirely optional (actually, I know very few people that go into these specific rooms, and those are usually the Perfecter game players like VanitasXII for example), enemies are common but not particularly strong. There are several moments where you can recuperate with little to any drawback, and the entire dungeon is basically a trial and error scenario. Which is good, considering it is the very first dungeon in the game.