I'm using nmap and I am having trouble finding which ports are dangerous and pose the greatest security risks.
Which ports are potential security risks for nmap?
I was thinking samba, telnet, vnc and ftp will pose security risks but I'm unsure.
It doesn't really work like that.
Ports don't pose a security risk in themselves; it's the application listening on a port that has the risk attached.
And, how vulnerable any given application might be depends on many factors - is the application well patched? Is it well configured? So you can't just say "VNC is a risk", what VNC server is it? What version? How is it configured?
And, of course, security is always more complex. For example, I bet you picked telnet because everyone always says "telnet's insecure, use SSH instead", right? Which is true, but that's because the protocol telnet uses isn't secure against eavesdroppers, it's nothing to do with there being exploits for telnetd (i.e. the application that typically listens for telnet connections).
What you're meant to use nmap for in this context is to check that only the applications that need to have a port open actually do have a port open.
If a host doesn't need sshd, but nmap finds one listening, then you know to go onto the host and fix that. Which is nothing to do with how secure sshd or ssh is.
All open ports providing services are a potential security risk. The nature of the security risk depends on nature of the service that a program provides. Services providing Samba, VNC, FTP, SSH, HTTP, ... protocols all pose a risk to your security, each in its own (but not unique) way.