The Mirror Shield is a iconic item (that will always change visually) in the Zelda series. One of my personal favorites of course, though I admit there's not many foes to test it out on.
My first mirror shield was in Ocarina of Time in the Spirit Temple. They had a couple great puzzles revolved around it.
The boss battle against Twinrova was beyond awesome. I always like a shield that can reflect fire and ice and send it right back to it's caster.
I liked the one in Majora's Mask as well, though the memories were a tad less thrilling. Spooky face on this mirror shield.
The mirror shield in Wind Waker was a bit forgettable, though I did like the visuals of reflecting light due to it's cel-shaded graphics.