I loooooove monsters.
So let's talk about designing effective monsters. The same fundamental rules apply whether you're designing for a book, an RPG, a TV series with no end, a horror movie... well, there are differences, but let's talk about the parts that are the same.
The first is that a monster has to embrace a part of the human condition.
That sounds pretty hoity-toity, but it simply means that the monster has to feel like it means something, like it ties in with the world and the people in the world.
For example, a werewolf is about rage, losing yourself to bestial instinct. It's fundamentally the same monster as The Hulk.
A vampire is about obsession with death, something most humans can understand pretty easily. These days there's incarnations that are more about obsession with hidden worlds, but classically it's about death. Incarnation, get it? HAHAHAHAHAHAAaaaaaa... sigh...
The daleks are about obsession with perfection, with purging imperfections. Actually, many old Dr Who monsters are like that, because at the time the reigning monster concept was based on Nazis and WWII. So they resonated very strongly with the times.
These monsters are powerful monsters, assuming the audience understands those parts of the human condition (from either side). These monsters can be used to tell stories about those parts of the human condition. Vampire stories about running from, defeating, and/or accepting death. Werewolf stories about the price of controlling or failing to control yourself. Dalek stories about being a machine... or fighting against one.
So, to be clear, a monster is a window on the human heart. A monster cannot just have more HP or a deadlier attack. If you make such a shallow monster, it'll never catch anyone's attention.
Some people say that a monster is like a funhouse mirror of the human heart, but I think that's more limiting than the idea of a window... because you can see through a window in both directions.
Anyway, if you want your monster to catch people's attention, there are a few additional things you may want to consider.
The first is what powers the monster has.
This is fairly obvious - monsters have powers. Jason lives through everything, the alien queen is both big and nasty and, if you leave her alone, will fill your planet with horrors. Werewolves regenerate and have the strength of a wild animal. Vampires can hypnotize you, disappear and reappear...
The powers of a monster should generally support their high concept. A werewolf with the ability to hypnotize makes little sense, because there's no connection between losing yourself to bloodlust and going "oooh, you're getting sleeeepy". Doesn't make any sense.
In general, powers are pretty flexible. It's not that the fundamental power has to perfectly match the concept... it's that it has to be able to match the concept when deployed. Both vampires and werewolves regenerate, but they regenerate in very different ways. Werewolves regenerate constantly, driven by a constant, frothing feral anger. Vampires classically regenerated with the fall of night, integrating them into the world's own concept of life and death.
Weaknesses are the same, but are usually tightly linked to the powers of the protagonist and the actual specific story you're trying to tell. Daleks can only be defeated by screwing up the cogs of the machine - trying to face them head-on is a nightmare. Werewolves can only be defeated by the metal that represents purity - silver. Fairies can only be defeated by humanity's greatest crime against the natural world - worked iron. Vampires can only be defeated by dealing with the organ that represents life, but has been perverted to fuel their undeath.
It really does depend on the story you want to tell. Sauron's weakness involves hiding behind rocks. The weeping angels have the weakness of being looked at. These are, when stated baldly, really silly weaknesses. But they match the powers of the characters in the story.
Thirdly, presentation does matter. Werewolves are unpopular because they're overdone. Weeping angels are popular because there's weeping angel statues everywhere and most youngsters aren't religious enough to have assigned them any other connotation. Hijack it and make it a monster!
Well, that's a kind of art I don't have much advice on. It really depends on your audience - what one audience finds scary, another will find campy or even cute.
Fourthly, consider whether your monsters can be expanded upon without diluting them. Can people tell other stories with these monsters? Or are you telling just about the only one?
Even if no other story is ever officially told, many audience members will happily daydream or have nightmares about other stories they make up for themselves. So it's best if a monster is somewhat flexible.
Let's use two Dr Who monsters as examples:
The daleks are a bit passe today because they are based on our parents' or even grandparents' experiences. But, fundamentally, they are a very flexible monster that can be used in a lot of different stories. While there have been good stories and bad stories, the concept of "dalek" comes out the other side pretty well, ready to be used again. Even though they look stupid!
On the other hand, the weeping angels were a devastatingly cool monster when first introduced... but the story that was told was basically the only story that could be told. So, to bring back the cool monster, they added in new powers, new needs, new ideas. And basically ruined the monster - it's a convoluted mess of powers that don't really embrace any one human element.
The angels were a bit of a limited monster in the first place, because their powers and weaknesses were designed to tell that one story. Within that framework, they were nightmares. They represented fear of the dark given perfect form: not just fear of night, but fear of every moment of darkness, even the moment in which you blink.
But their powers were extremely specific, and that means they can only tell very specific stories. Their powers weren't generic or varied, like vampires and werewolves and daleks and time lords: they had a few super-specific powers, and that was it.
The monster wasn't made with an eye towards expansion.
There's nothing wrong with that, if your idea is specifically to tell that one story. But if you're introducing a monster you want to be flexible enough to use in several very different kinds of arcs, you need to start right off the bat by considering whether it can be expanded without dilution. Can powers be gracefully added or ramped up? Can weaknesses be altered or shored up? Can all of this happen while the monster still represents some facet of the human heart?
Those are the considerations I put into monsters.