Great poll idea! But on topic I think Forest Temple from OoT. This dungeon was one of the best in the game, and no other forest dungeon has really been that fantastic. I really hope there is a great one in the next Zelda title.
And something I want to say to Nintendo: Mix it up a bit and PLEASE DON'T make every game have the earliest dungeon be forest themed. Its been a real occurrent theme. It's not that hard, just mix it up a bit and have a forest dungeon 5th or 6th instead of 1st or 2nd.
(I know there's a lot that aren't exactly 1st, but on the home console titles, since OoT, its been: <OoT> 1st=Deku Tree, 1st of adult dungeons=Forest Temple, <MM> 1st=Woodfall Temple,<WW>2nd=Forbidden Woods, <FS> 1st Real Stage= Sea of Trees, <MC> 1st=Deepwood Shrine, <TP> 1st= Forest Temple, <ST>1st= Forest Temple, <SS> 1st=Skyview Temple..... Notice the theme?
Only forest dungeons not 1st or 2nd are the Wind Temple in WW, which is 5th in the game, but is more wind dominated with a setting of a forest, and Skull Woods in ALttP, which is supposed to be the 7th dungeon, but this one really is only forest because of where it is and the vines on the walls.