My favorite in Ocarina of Time was the Iron Knuckle as well. Of all the enemies in the entire game, they were one of the funnest and most challenging enemies to fight, and the appearance of one always promised a cool battle. Their speed was unimpressive (which means you can be really cheap if you want to kill them easily), but it was partially compensated for by their increased speed after shedding their armor, although this was only true for 3 out of 5 of the game's Iron Knuckles (the one just before Twinrova, and the two in Ganon's Tower). And yeah, they were definitely brutal on a 3-Heart Challenge, because they kill you in one freaking hit. >.<
Would like to see the Iron Knuckles again, actually, as opposed to the Darknuts. Darknuts are cool but I always like the Iron Knuckles better, ever since I saw them in Ocarina of Time. Would like to see a new version closely follow OoT's, unless it was going to try something new.
An appearance like OoT's could be possible in Skyward Sword, given the cultural imagery we've seen so far in the trailers.