Which is you most important system?
Many organization would answer Active Directory. A lot of applications and systems are depending on Active Directory, for example Exchange. Most of the regular issues within Active Directory can be handle if a administrator get notice early. AD management pack monitors all core components of AD, client services, replication and gives you a good overview of your directory service health. There is one disadvantage, AD MP does not monitor security, for example changes in your domain admin security group.
I have written a article about how you can monitor changes in some key security groups. This is not a complete management pack, but I will show you how easy it is to create some basic rules.
You can download the article here