The only boss that puts of a little challenge in those games is Gyorg.
However, the other bosses aren't bad, just too easy. There are several nice boss fights, but they should increase the difficulty in them by increasing he speed and damage of the bosses. ^^
OoT: Probably barinade. It's one of the few bosses that can't be killed in a few seconds, and there's no easy trick. Except maybe killing multiple jellyfish with deku nuts.
MM: Gyorg. You have to get in the water to damage it, then get out as fast as possible to keep from getting killed. Though I remember one time I stayed in the water as a Zora, and swam in circles trying to stun it. It worked at least once. Still, it's a hard boss.
Gyorge was hard...I can't remember how many tries it took me to kill him. He, or would it be a she since it had babies?, always ate me.
If Dark Link in OoT can be called a boss, he had to be one of the hardest; i had to use din's fire most of the time.
Sometimes I have trouble getting the opposite elements to hit the Twinrova sisters in their first form. Ugh! One day I was able to do it just fine then the next i was all horrible at it haha.
Gyorg for Majora's Mask =\
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Most of us seem to find gyorge hard; so he should be declared a hard boss . Odolwa seemed pretty easy if you figured out that the moths and the bugs are attracted to bombs...would dark link from OoT be considered a boss?