Ask HN: What's the best remote desktop viewing software for Windows?

I am looking for a remote desktop viewing software we can install on a few dozen machines. UltraVNC works but some people have suggested a tool called Radmin which actually performs better (better screen resolution and faster refresh rates). Ideally I would like something that has a portable install because it will make automated deployment easier.

Windows Remote Desktop won't work because it actually takes over the session. We need a real viewing software that doesn't change anything on its target machine.

Any suggestions? Are there better VNC versions than UltraVNC?