Top 1: Cobalt 2
For me, this is the far best theme for Sublime Text. Beautiful colors which help you, capture the source code fast. The sidebar icons are really beautiful and are making it easy to determine file types pretty fast. The hightlight color is a rich yellow, a great color for the ui to focus fast.
By the way: There are also Cobalt 2 themes for Alfred and iTerm!
Top 2: PreDawn
I used PreDawn a long time, until I a stumbled upon Cobalt 2. The highlight color is a bright orange. The colors are chosen very well and make it easy to scan the code. Unfortunately the last version I used did not have a lot of sidebar icons.
Pro: There are a lot configuration parameters to customize that theme, like preferences for things like tab sizes.
Top 3: Spacegray
Spacegray is, as the name may say, not that colorful as the other two. But the color choice is great, too. The theme as two modes you can choose from, dark and light. Unfortunately there are no sidebar icons. But beside that it is clear and good to use.
If you just want to try a theme and don't want to download it for that purpose, you should have a look at ColorSublime. The plugin works like the package manager. The great thing is, you can browse through the theme list in sublime and see a live theme preview with your current code open.
All images are from the repositories of the themes