Celebrim
Legend
In a post 1st edition environment, this is to me really an unanswerable question. No monster type can be refered to in its totality any more, if indeed they ever could. Asking 'what monster is most powerful' is really a campaign specific question because any monster can be advanced in hit die, given templates, given class levels, and/or given a higher point buy, and be made into something that the PC's will find truly terrifying.
For instance, Advanced Fiendish Half-Dragon (Black) Paragon Lenerean Pyro-Hydras tend to strike notes of fear in most anything.
So, really, we might want to ask 'What non-unique monster first printed before 1982 (does that cover all the classics?) do players least want to face.', but even this question depends alot on which edition of the game we are talking about.
First edition players had a particular dislike for Will-O'-Wisps, and a generally disliked meeting Beholders, Black Puddings, Shambling Mounds, Mind Flayers, Rust Monsters, and Thesselhydras. Which one you disliked the most depended alot on personal temperment and class. Dragons tended to be one shot wonders (BREATH WEAPON!!!) and were feared because thier high end damage generally exceeded everyone's hit points but maybe the fighters, and if you couldn't take them down quick, someone died. Vargoille remained intensely disliked regardless of level. Cloakers could be quite powerful if they attacked in numbers. Rot Grubs, Green Slime, and Yellow Mold varied from insanely lethal to annoying depending on how they were used.
Virtually everything else that was particullarly feared after level 9 had class levels - Drow, Liche, NPC's, Shades, Vampires (with spell casting abilities), etc.
For instance, Advanced Fiendish Half-Dragon (Black) Paragon Lenerean Pyro-Hydras tend to strike notes of fear in most anything.
So, really, we might want to ask 'What non-unique monster first printed before 1982 (does that cover all the classics?) do players least want to face.', but even this question depends alot on which edition of the game we are talking about.
First edition players had a particular dislike for Will-O'-Wisps, and a generally disliked meeting Beholders, Black Puddings, Shambling Mounds, Mind Flayers, Rust Monsters, and Thesselhydras. Which one you disliked the most depended alot on personal temperment and class. Dragons tended to be one shot wonders (BREATH WEAPON!!!) and were feared because thier high end damage generally exceeded everyone's hit points but maybe the fighters, and if you couldn't take them down quick, someone died. Vargoille remained intensely disliked regardless of level. Cloakers could be quite powerful if they attacked in numbers. Rot Grubs, Green Slime, and Yellow Mold varied from insanely lethal to annoying depending on how they were used.
Virtually everything else that was particullarly feared after level 9 had class levels - Drow, Liche, NPC's, Shades, Vampires (with spell casting abilities), etc.