Mild Hyperion spoilers:
The Ousters in Dan Simmons' Hyperion. The Ousters themselves were once human, but genetically modified themselves to better fit all manor of environments, including the high radiation and hard vaccum of space, or the intense weather/gravity of Gas Giant planets. They also formed themselves a coalition with all the other sentient races of the galaxy.
Nearly done The Fall of Hyperion, I have to agree with this one majorly. The Ousters in-and-of-themselves are just an awesome idea for a future humanity, really different from anything I've see in sci-fi before (though I have admittedly read very, very little sci-fi). The way Simmons handles their introduction and gradual reveal is part of what makes them so cool.
Now, despite only reading Ender's Game and half of Speaker for the Dead, I really like what I've read online about Orson Scott Card's aliens. The guy's a total douche but I must admit that I think his aliens are great, and my reading about online (wiki's, other people's accounts) actually influenced my own alien-design quite a bit.