The most complex invert on earth is the stomatopod, or mantis shrimp. I find them the coolest for the sheer number of capabilities they poccess that modern science sometimes incooperates!
Having the ability to see Infra-red, and Ultra Violet lighting sets them apart from any other animals on earth.. they have the #1 most complex, and best vision out of anything. I know it was a joke in a video, but like the guy in it quoted, "Imagine a color you could never imagine.. now do that 8 more times, that is how the mantis shrimp do". Pretty mind blowing.
There's also the more well known capabilities, such as the strike acceleration of a .22 caliber bullet, and the
rumor that they can break glass! (Dr. Caldwell said this happens less than once in a lifetime with large smashers.. but still possible!)... and of course the diversity in them, not all of them share the same characteristics as the other species.. They all have different behavioral patterns, alongside their own dog-like personallities when interacted with.
I literaly could write a 9 page essay (with better paragraph structure, I know

) just talking about 1 thing a mantis shrimp can do.. and they can do ALOT.
I know I wrote that a little sloppy and sound like an over excited child, but it's just so difficult to emphasize properly how awesome and superior the stomatopod is to any other known lifeform.