The Nexus 5.
I seriously hope that the new Nexus does something different on the design front. I started my Nexus journey with the Nexus 3 (SGN), the curved glass and the tear-drop design was quite an achievement in terms of looks and ergonomics. The Nexus 4, the curved edges is a nice touch, but i expected more. The gorilla glass at the back with the new hide and seek glitter imprint is also nice, but you end up covering those design bits with shells to avoid the glass getting scratched.
I really hope the Nexus 5 is slimmer, weather proof (water and dust resistant). Has some new innovation on the display side, maybe the best possible implementation of OGS (one glass solution) that makes content appear as if it is floating on the glass. Basically make the glass/screen as thin as possible. Also, it'd be nice if it can borrow some goodies from the Moto-X, an always on/active/listening assistant that responds to voice, not needing you to touch the phone for most common actions.