pool.ntp.org servers. Seems most NTP server ignore the telnet command.
Then I thought I'd use net time, but it has been deprecated in Windows 7 in favor of the newer w32tm command.
There's a litany of switches to use with it, but the /monitor /computers:server are the ones you want.
And using the command: c:\w32tm /monitor /computers:time.nist.gov
You will get the following output, showing it works:
ICMP: 59ms delay
NTP: -0.1402919s offset from local clock
RefID: 'ACTS' [0x53544341]
You can find a huge amount of popular public NTP servers at NTP.org