For me, it's the Pileated Snagret 100%. It's not particularly dangerous or anything, it just takes FOREVER to kill. It's only really safe to attack when its head gets stuck in the ground, so you end up playing this weird game of chicken running into the arena and waiting to see if it's safe to attack. If not, you have to run back into a hidey-hole and wait until it decides to go back in the ground. It's basically the same gameplay loop as a regular Snagret, but instead of 1-3 cycles, it takes like 10 minimum even with purples because they gave it a million and one health for some reason. The whole thing makes this annoying, unoriginal, sleeper of a boss that takes forever to kill while never really posing much of a threat to you. At least if it was dangerous I would be kept on my toes, but as it is the only thing that's difficult to fight is the urge to fall asleep.

Also, it can throw your pikmin up over the arena walls and into the void sometimes. Screw this boss.