Hi, I can't say for sure what the best is, but I use Apple Remote Desktop 3.1, it's made and sold by Apple, so can be purchased from the Apple website.
In some ways it's very advanced, you can view other computers screens, and see what they are typing, you can take control of their computer, you can ask it to scan your network to see if anyone has plugged in another machine, like a laptop, it also allows you to distrubte tasks to all the computers, like install latest updates from Apple.
It has a home screen which will list all the computer on the network and say what they are doing, who is logged in what applications they are running etc.
You can send Unix commands as well, like shutdown or logout all computers.
I know you don't have to install the software on all machines, depending on what you want to do. As you just need to install it on your machine to see what everyone else is doing, and control their computers, but if you want to send tasks and Unix commands then you would need to install it on all the computer on you network which you wanted to send tasks and commands to.
The only drawback is the price. £199 for 10 clients, £349 for unlimited clients. I happened to get mine for free.
here is the link to the apple store for Remote Desktop:
http://store.apple.com/Apple/WebObjects/ukstore.woa/wa/RSLID?mco=9401CC05&nplm=M A231
An alternative would be someting call Chicken of the VNC, I have no idea how it works, or what it is like but it's free available to download form Apple,
http://www.apple.com/downloads/macosx/networking_security/chickenofthevnc.html
hope this helps, if you need any more info please don't hesitate to e-mail me at louishoward@mac.com