Same as most. Twilight Princess' style is amazing. It is by far the greatest way for a Zelda game to look. I don't want the graphic's/realism to go any further. Maybe just some very detailed textures. That's all.
As for my least favourite, I really have no clue. I can't really comment on the 2D ones because they don't really have a style they follow where as 3D titles really go out of the way to change it up and follow a particular style.
It's a pretty tough one, but I would probably have to go with Skyward Sword.
Odd as it may sound, I like both TP and WW styles. The OoT 3D style however, it appears too...mixed, its like TP background and characters with WW textures and rendering.
OoT and MM are definitely my favorite styles. I feel like they are just a perfect fit for the series. I dont really like the WW, Spirit Tracks, or Phantom Hourglass style. Makes it just seem less serious. Skyward Sword looked good and I did like the darkened graphical style to TP.
I prefer the OoT and MM style but detest the WW cell shading. Although, I also liked Wand of Gamelon's style. Zelda's Adventure had a nice realistic look for a top down game. And yes, My uncle bought them when they came out when I was a kid, so I played wand of Gamelon and the other that goes with it. Watched him play Zelda's Adventure - or rather I watched him struggle through it lol.