It's less 'top 20 ports' then basic knowledge about a number of common services, including the port number they happen to use. TCP or UDP is also good.
HTTP(80), HTTPS(443, 8080), SNMP(161), FTP(20&21), DNS(53), POP3(11), SMTP(25), Telnet(23), SSH(22), IMAP(143)
IMAP isn't that common, but you can substitute DHCP(68), IRC(194), etc...