All depend on what you want to spend. My first choices are Herpstat or Helix, both are top of the line and more expensive. Next I would consider Johnson Control or Ranco ETC-111000. These are both good, just operate a little different than the other two I listed. And these last two make clicking sounds when they turn off and on while keeping the right temp.
Here are a couple sites for you to check out.

http://www.reptilebasics.com/thermostats/
http://www.spyderrobotics.com/index.html

Hope this helps!