What are your favourite fictional & constructed languages?

We've all been talking about how to make your own language and it seems like a hard thing to do, so let's bring in other people's examples to show that it can be done!
What makes your chosen example great and how has it been used? Let's see how we can use them to tell us what to do and for our own inspiration.

Whether it's as well known as Klingon or as obscure as Hymmnos, anything is welcome!

Hi! I made a language called Rutavk, and I'm having trouble working it out. my name is Paul Tamez or Roin Wopuc