Mirror Shield from the Wind Waker for sure.
I also like the Twilight Princess & Skyward Sword Hylian Shields (2nd place tie between those 2) but Wind Waker Mirror Shield is definitely #1.
Maybe I'm just partial to Majora's Mask, but I really loved the Mirror Shield from it just because of how it looks, even though it does look kinda say and creepy.
My favorite shield is most definitely the Hylian shield. I remember when me and friends would pretend to be Link and I'd rip them off by selling overpriced Hylian shields. Fun times.
Mirror Shield from Ocarina of Time was pretty awesome. I liked the Mirror shield in A Link to the Past aswell, just because it was so huge compared to Link
I guess the conclusion is that the Mirror Shield is the most awesome, but the Hyrule Shield is always awesome aswell. It looked great in Skyward sword aswell. Did the gauntlet just to look cool with it
Either the Mirror shield from MM or the Deku shield from OOT. I know that the deku shield wasn't very useful since it could be burned, but I like the deku emblem.