I've recently been giving some thought on the various monsters that have appeared in Doctor Who, and I've begun to wonder what exactly are the keys to making a monster successful.

Creatures like the Daleks and Weeping Angels were only supposed to be one-shot monsters, but exploded into popularity. Despite having only one televised appearance early in the Fourth Doctor era, the Zygons are one of the most talked about Classic Monsters.

On the other hand attempts at creating something with the intention of being popular have fallen flat, such as the Voord, who no one seems to remember, and the Slitheen, who have never been taken mocked by the fandom.

Why do some monsters end up being immortal symbols of the show, while others fade away into obscurity?

What makes a good monster good, and makes a bad one bad?

Something that's genuinely scary, sinister and menacing.

Good question, I'll have to think about that but I do think we need a humanoid monster as a regular adversary.

Mystery is a very important element IMHO.

In the very first dalek story we only got very brief shots of daleks at first, and what was inside a dalek remained unknown for years.

Similarly, in the first Alien film we only get short shots of parts of the alien, so even its fundamental shape is unknown, so it's terrifying. In the later films it's just monster shooting and the aliens aren't frightening at all.

Villains are different though. A 100% evil villain is boring. A villain who has some element of good generates some doubt as to how they will act, which is interesting. I can't think of an example in science fiction of the top of my head, but Joan Ferguson (the Freak) in Prisoner Cell Block H is an excellent example.

Design is crucial.
There have been some utterly brilliant monster designs - not least the totally unique Dalek shape which was so good that it remained virtually unchanged for nearly 50 years - and the current production team have clearly accepted that the redesign (which changed the basic shape) was a failure. The original Cusick design was THAT good.

Other cracking designs - Ice Warriors, Ood, Draconians, Davros (original Wisher mask, and the recent Bleach version) Sea Devils, Ogrons, Exxilons, Autons, original Silurians, Tenth Planet Cybermen (virtually all the subsequent designs are just too much like generic robots), Sontarans, Slitheen (yes, really, they actually LOOK great, it's the farting and attempts at comedy that let them down) and top of the list, as always, the Zygons.

Failures would include - Mandrels (which look like a Who Monster as drawn by a five year old), Nimon, 80s versions of Silurians, Sea Devils, Sontarans and Davros, Voord.

It's interesting that ones I consider to be failures often stem from stories that are written, produced and acted with little conviction - there has to be a link. If the acting and writing is good enough it's usually possible to overlook some of the deficiencies in Monster design.

As I found in the Zygon DVD thread, the short answer for classic Who seems to be, "is John Friedlander in the credits"

Must mention The Satan Pit from the new series, as it was the first to have the Ood (who I love) and it had the Demon/Devil which was so well done.

The concept of the Cylons in the rebooted Battlestar Galactica is a good example, as at times some of their allegiances and motives are unclear and there is a fair bit of side-swapping. Gaius Baltar from the same show is another one.

Edit: and of course, Tom Zarek

Personally, I don't find the classic monsters, like the Daleks or Cybermen remotely frightening.

I think they are as product of their time. Both had an 'inhuman' desire to kill - and both were essentially a Nazi metaphors, which was something still very fresh in the public consciousness in the 1960's. But both have a clear goal to fulfil which never changes - and thus, both are routinely anticipated and easily dispatched.

To me at least, the element of unpredictability is crucial in creating a classic modern day monster.

The Weeping Angels for instance - want only to feed. It's this primordial need, reduced to it's basest, instinctual level which makes them a force to be reckoned with. They are pure predator. You can't argue with it or reason with it. It's just pure, primal hunger staring you in the face.

But you never know if it's a statue or an angel till it's too late. Nor how much it wants to feed from you - nor where you'll end up. It's the unknown. You're totally in the dark.

And speaking of which, the Vashta Nerada took our primal fear of the dark and left you guessing... Am I stepping into a Shadow - or a Ravenous Swarm? Again, it's the unknown.

Likewise, the Silence was a brilliant villain. Their motive was never clear - and if you ever tried to get away from them - you'd immediately forget to flee. You became your own worst enemy, in effect. Watching them always reminded me of a nightmare - with a boogeuman you could never outrun - or a shark you could never outswim. Turning off your instincts leaves you utterly at their mercy. But again... What do they even want? Even if the told you -byou still wouldn't know. Again, the elements of the unknown which totally sh!ts people up.

A Farting Strip-Mining Teletubbies with Sloth Claws, frankly ain't gonna cut it nowadays.

The Voord never had a chance because Dalekmania was already happening around them... also because they were rather peripheral even to The Keys of Marinus, only appearing in the first and last episodes. I don't think they were a rubbish design as such.

Fear of the unknown is an extremely powerful tool in creating tension I think. For example, it took many episodes before the true nature and threat of the Shadows became apparent in Babylon 5.

I love the Sea Devils; probably a combo of being scared of them as a child, and then reading a lot of HP Lovecraft as a teenager, and kinda connecting the Sea Devils with the Deep Ones.

Decent writers who understand the characters, their strengths, weaknesses and motivation. I mean how can you have the daleks be impervious to all physical and energy weapons yet have the doctor constantly defeat them. What do they want? Does it seem like the daleks would be bored if they exterminated all other life?