Definitely Malboro. I remember the first time I went to The Island Closest to (Hell or Heaven, cant remember which) and I had heard about how tough the monsters on those islands were and sought to fight a few of them, first was a hex dragon, I took him down easily enough... then it happened, Malboro came on the scene and Bad Breathed me... then proceeded to rape my poor heroes. I lost 3 hours of gameplay that day.

Tonberries on the other hand... they were pretty easy to beat, sure they have the death attack, but you get so many turns in before they close in on you it renders them mostly harmless.