For day-to-day use, all are quite secure. Lion is obviously most secure, but as long as you exercise reasonable common sense, you shouldn't have any problems with any of the 3 versions of Mac OS X.
Make sure your firewall is enabled.
In Safari Preferences, make sure "Enable Java" is unchecked.
In Safari Preferences, make sure "Open "safe" files after downloading" is unchecked.
Make sure you exercise care in where you get software you install.