Let me clarify on my reasoning (I voted Bowser, btw):
Piranha Plant: it's overly simple, though the easiest to make a stupid mistake with the B button (grab/release). The tactic for 3rd life point is stay in a corner when the seeds come up, and stay there until a seed is spat at you. Grab it, and then the usual.
Hammer Bro: be wary of irresponsive DS screen, that's it. (The most pushover-y "battle". It would be better if the sequence was 3, 5, 7)
Dry Bones: too much luck involved to get a short time. Dry Bones isn't very intelligent either, and the third life point leaf that blocks a button in the top screen is useless...
Kamek: the first 60-70 seconds are a waste of my time. Battle strategy against the Magikoopa himself is OK, but I hate waiting for the books to clear my path (they can hurt you, those books when Kamek stomps like crazy after being inked).
Bowser: the only true boss battle. The first form's time can be cut to around 1/3rd at the cost of 1HP (I won't say, though). Second form is the hardest to pull off without 5 seconds of waste. Third form looks hard, but can be done in one swoop at the cost of 1HP again. Though if you're a conservative player, Bowser's strategies let you exploit that too.
So yeah, Bowser is the best boss for legitimacy as a battle against a boss. =P