Best place to define plugins' constants?

[ccagle8]

Another idea is: use only constants for default settings and every setting is changeable using an administration page for the plugin and the settings are stored in a file in data/other. If there is no settings file, the defaults are  used.

I like this approach, no matter how small the plugin and/or "default setting"... but if that isn't an option, editing gsconfig.php would be a close #2 alternative