Three themes which looks better than default Gnome Shell theme: XGnome Enhanced, Viva, and Adieze (PPA)

Install XGnome Enhanced, Viva, Adieze and Zhanshi-glass themes in Ubuntu 14.04 Trusty/13.10 Saucy/Linux Mint 16 (Gnome Shell)/other related Ubuntu derivatives

XGnome Exhanced is highly customized version of default Gnome Shell adwaita theme and it looks minimalistic, modern and clean., it is available for Gnome Shell 3.4/3.6/3.8/3.10. It seem better than default Shell theme and for better results Sony-Sketch fonts are recommended to use with this theme.
Viva theme is Gnome's default adwaita theme, which has different color combinations. Adieze Gnome shell theme is mixture of Adwaita and Frieze.
Fs-icons used in screenshots, also check icons collection. You can use Gnome-tweak-tool to change Gnome Shell themes (how to enable themes for Gnome Shell).

Adieze Theme

Viva Theme

XGnome Enhanced Theme

To install these themes in Ubuntu/Linux Mint open Terminal (Press Ctrl+Alt+T) and copy the following commands in the Terminal:

Terminal Commands:
sudo add-apt-repository ppa:noobslab/themes
sudo apt-get update
sudo apt-get install adieze-gs
sudo apt-get install viva-gs
sudo apt-get install xgnome-enhanced-gs

That's it