I actually didn't care for any of them much.
Octo Oven was probably the best. Solid design, lots of detail. Easy, but it was the first boss after all. It could have done more, though: the second phase wasn't really different from the first, and the third phase has attacks that literally do nothing (like when it "swings" the bread at you, it always attacks in the wrong direction).
Octo Samurai: Nice design, but hindered by the fact that you can just go up to it and shoot it. Absolutely trivial in difficulty and you can skip most of its attacks if you're aggressive enough.
Octostomp 2: First phase is again trivial, far too easy this late in the game. I did like the second phase. Third phase is just ridiculous, too simple of a pattern, and not well balanced for different weapons.
Octo Shower: If the use and placement of the Rolonium isn't a hint that this was badly designed, I don't know what is. Very frustrating chasing it around and trying to figure out how to hurt it. My best time was actually with the Brush because it hung around the center almost the whole time.
Final boss: Coming from Splatoon 1, this was a disappointment. Each phase is unnecessarily long for the lack of variety in attacks, and there's so much RNG involved in when he makes the one attack you can counter him with. If the game had had more plot going up this point, the script would have been a plus, but it didn't, so it fell flat and was just silly. Plus the final phase, which is the same (minus RNG) each time you replay it. And it's far too easy--I only ever died because I was blinded and distracted by the laser show.
