There was a controversy some time ago involving D-Link and Poul-Henning Kamp where the former were using the timeserver set up by the later as default in their routers, effectively generating a DDoS on the server without giving any recompensation for it. The matter was amically resolved in the end, but it seems that corporations didn't learn anything from it. I was browsing today through the configuration of an Edimax router and much to my surprise I saw the default timeserver being set to 18.104.22.168, which seems to be a timeserver at the University of Michigan (judging from the partial reverse DNS). I doubt that they have any agreement with Edimax!
Now for the question of which NTP server to use? You can use pool.ntp.org which resolves in a round-robin way to NTP servers which are voluntarily publicly available. There are also subsets available by continents and by country, so that you can choose a server which is nearest to you and use the other servers as fallback. To get a more detailed description, visit the pool.ntp.org website. And make sure that you check your routers default settings!
PS. Some el-cheapo routers (like this Edimax) do not allow the setting of NTP servers by name, only by IP. In this case you should use nslookup to
randomly choose a server from the pool and set it as your NTP server.