gistfile1.txt

We have the following in /modules/manage_windows_feature/manifests.init.pp

define manage_windows_feature($feature_name = $title) {
  $powershell_cmd = "powershell.exe"
	$exec_policy = "-executionpolicy remotesigned"

	exec { "install-feature-${feature_name}" :
		command => "$powershell_cmd ${exec_policy} -Command \"Import-Module ServerManager; Add-WindowsFeature ${feature_name}\"",
		path => "C:\\Windows\\sysnative\\WindowsPowershell\\v1.0;${::path}",
		onlyif => "${powershell_cmd} ${exec_policy} -Command \"Import-Module ServerManager; if((Get-WindowsFeature ${feature_name}).Installed) { exit 1 }\"",
		logoutput => true,
	}
}

This is used across a number of our modules and called from within a module (/modules/windowsfeatures_iis_application_development/manifests/init.pp) and it looks as follows:

class windowsfeatures_iis_application_development {
  manage_windows_feature { 'Web-Asp-Net': }
	manage_windows_feature { 'Web-Net-Ext': }
	manage_windows_feature { 'Web-ISAPI-Ext': }
	manage_windows_feature { 'Web-ISAPI-Filter': }
}

this manage_windows_feature is used across a number of classes. Therefore, where is the best place to keep it?

Exactly where you've got it - in it's own module.