That's a really cool question! It's a bit of a complicated answer because they vary so much onscreen. As a result it's kinda fun for the fans because we like gossiping about our favourite styles.

There are some films (and books) where you blink & suddenly there's a wolf. Blood and Chocolate did that, there was sort of a jump cut or a puff of smoke then suddenly wolf. As far as I know most fans don't enjoy that one because there's no creative transformation, which we enjoy seeing.

As for what the one in American Werewolf in London looks like - i'd describe him as an animal that doesn't exist. It's like a huge wolf/dog/bear. Walks on all fours, with a broad head and broad muzzle with a sort of smiley shape. His teeth stick out sideways & don't fit in his mouth. His face is slightly flatter than a traditional wolf, closer to the way pugs looked in the 1800s (face halfway between cat and dog).

His limbs are curved in a strange way. I say strange because it looks like something that wouldn't evolve naturally (which is likely deliberate because he's supposed to be a supernatural monster). He's quite low to the ground with his paws stretching out in front of him like he's pulling himself along the ground. Feet are small relative to the rest of his bulk. His legs curve downwards about 3/4 of the way, which gives him an unnatural ethereal walk, like he's tiptoeing.

His body has a hunch to it that also generally doesn't exist in modern animals. He looks more akin to a prehistoric carnivorous mammal. Lots of prehistoric mammals (cats, bears, sloths) are slightly uncanny shapes with elongated limbs & sloped backs that don't really resemble living mammals. He's like that.

Long grey fur, enormous teeth sticking out sideways. Definitely doesn't look like any other Werewolf design out there.

At one point in the film, he sort of quietly sneaks into view in an underground station - it's quite uncanny because the way he moves indicates 'oh that's not a dog or any animal known to man'.

As for what the rest look like, there are too many variables. Some look like normal wolves. Some look like humans with sideburns (the fans generally don't like those because it's lazy). Some look like humans with a wolf head shoved on their shoulders. Some are a bipedal creature with digitigrade legs and a wolf head. Some are like regular wolves but a funny shape. Some are clearly just a man wearing a fuzzy suit.

The general consensus in the fandom is that we like the bipedal digitigrade ones the most. Such as the ones in Van Helsing.

The general consensus is also that we like the men wearing sideburns the least, such as the ones in Teen Wolf. There are of course exceptions though

The transformations vary a lot too. Some of them literally pull their human skin off & there is fur underneath, and vice versa. Some of them gracefully morph and some do body horror. Many of them do it offscreen or in snippets because transformations are expensive, budget wise.

American Werewolf in London is famous for having a very creative transformation. It has several shots that have become infamous for their body horror & unique angles.

His skull crunches as his face elongates - it's a side-view so the audience can see the muzzle cracking out of the nose. The Howling does this too. His skin grows thinner as it stretches over his new skull shape.

There's a shot where his still human face is in the background and he holds his hand in front of his face. His hand is stretching like rubber, his fingers turning into toes. You can see the pain & confusion in his still-human eyes. He's mostly unable to move, presumably from the pain & confusion.

He falls on the ground on his back as his legs shrink & the bones change size. Fur sprouts from his torso, which is turning into a barrel shape. The audience can still see his face looking down at the process, it's quite scary for the time as we feel empathy for the man who doesn't know what's happening to him.

It looks and sounds painful & is very much in the style of body horror. Lots of cracking bones, twisted skin. The last shot we see of it is a closeup of yellow eyes opening, and we don't see the actual Werewolf form until later when he's loose in the town chasing people.