Depends on your budget. The best IMO are
http://www.coldbloodedsupply.com/pro...-SHIPPING.html
http://www.reptilebasics.com/ve-100
http://www.helixcontrols.com/DBS1000.htm
Decent on/off ones are
http://www.reptilebasics.com/ranco-etc-111000-pre-wired
http://www.reptilebasics.com/johnson...stat-pre-wired
And a decent cheap one
http://www.amazon.com/Hydrofarm-MTPR...9480702&sr=8-1
I use herpstat, johnsons, and the hydrofarm and like them all, herpstat being my favorite.