So here is a list of the five coolest Nudibranchs in my opinion. There are definitely more colorful Nudibranchs out there, but in terms of morphological weirdness/interest these are kind of at the top.
1) Tethys fimbria
|Photo cred Dominique Horst (www.bathymed.net)|
Creepy. Tethys feeds in the same manner, which would be cool enough by itself. But then you realize how BIG Tethys can get - almost 30 cm. Nudibranchs usually don't get much bigger than 10cm, and the colorful ones are tiny. 30 cm is massive for an invertebrate, especially for a shell-less organism like Tethys. Finally, the thing reminds me of a trilobite. Look at this video; coming up to it, it looks like we are entering a primordial coral reef. Interesting example of convergent evolution in terms of body shape (maybe?) who knows, maybe trilobites used their head to entrap small organisms.
|Valdes et al. 1998|
I've written about this nudibranch before (Dendronotus comteti), and I am still fascinated by its biology. It's no looker, but it is the only nudibranch to be found at hydrothermal vents (and I think the nudibranch found at the greatest depth - not sure about that one). Anything found at depth and in such an extreme environment is already of special interest, but to have one species representing an entire Order (which is a lot of species) is quite impressive. I believe that there must be more species of Nudibranchs down on hydrothermal vents/cold seeps/what have you, but so far this is all we have.
3) Hermosita hakunamatata
|Photo cred Peter Ajtai|
4) Onchidoris bilamellata
|Photo cred Me. Yay!|
5) Glaucus atlanticus
|Photo cred A.F. Julien|