So a kaiju (怪獣) is any kind of big, terrifying, and seemingly undefeatable monster in fiction. The kind we're most familiar with, like Godzilla, are daikaiju (大怪獣) which specifically refers to the giant kind. They vary from being fairly intelligent to being animals, and tend to attack cities and things, leaving devastation behind.

This is a pretty loose definition, though. So...

King Kong. Huge gorilla and protector of Skull Island. Otherwise, not particularly fantastical.
Is King Kong a kaiju?
Yes
67.2%
No
6.3%
Maybe?
14.1%
No but those big monster things he fights are
12.5%
Poll ended at .
In the Jurassic Park movies, the T Rex is pretty much undefeatable, apparently fairly intelligent, and good at destroying things.
Is a Tyrannosaurus Rex a kaiju?
Yes
26.3%
No
28.1%
Maybe when it was wrecking San Diego?
45.6%
Poll ended at .
The Ghostbusters did actually defeat the Stay Puft Marshmallow Man, but it was still a ridiculously big monster which wrecked New York.
Is the Stay Puft Marshmallow Man a kaiju?
Yes
87.9%
No
4.5%
No, too cute
3%
No, too puffy
4.5%
Poll ended at .

In pretty much any version of the story, the Tripods from War of the Worlds are basically unstoppable eldrich horrors which wreck everything in their path until they all get sick and die.

They're mechanical, but so is Mechagodzilla. They're from space, but so is King Ghidora.

Is a War of the Worlds Tripod a kaiju?
Yes
47.1%
No
9.8%
They don't count because they're vehicles
43.1%
Poll ended at .
The Reapers from Mass Effect come from space, are huge, practically invincible, and destroy pretty much anything which stands in their way.
Is a Reaper a kaiju?
Yes
51.4%
No
22.9%
They're... difficult to classify?
25.7%
Poll ended at .
In Star Trek: The Next Generation, the Crystalline Entity is an absurdly big living creature which roams around space indiscriminately devouring all organic matter in its path. Right up until its final appearance on-screen, it's basically unstoppable, leaving a trail of completely annihilated planets behind it.
Is the Crystalline Entity a kaiju?
Yes
52.5%
No
10%
This is a bit of a stretch
37.5%
Poll ended at .
The Transformers are big mechanical life forms which tower over city blocks. They're mostly only stoppable by other Transformers. And yes I'm using a screenshot from the movies, but take your pick of whatever version of them you like.
Is a Transformer a kaiju?
Yes
31.7%
No
31.7%
Maybe if it's a Decepticon?
9.8%
Only if it's a Dinobot
26.8%
Poll ended at .
Thank you for participating in this questionnaire about giant monsters 🦖
@InvaderXan this thread has made me rethink and broaden my personal definition if kaiju and it was a lot of fun. :)
@vicorva I just find them such an interesting concept!