You should select the time server that you want to use very carefully according to the following criteria:
The distance from your application server to the time server. If your server is in Europe, it would make little sense to select a time server in Tahiti. Always select a server which is not far away. This reduces the request time and overall network load.
How long it takes to receive the request is also relevant. Try different servers to get the best result. If you are requesting a server which is never accessible, you will always have an unnecessary delay.
Do not always use the same server. All time servers will lock out requests from servers that are flooding the server. If your application requires heavy use of time servers, you should consider one of the pools described later.
So where can you find a time server? Generally you can use any timeserver you can connect to. This can be a time server within your LAN or any public time server you have access to. If you decide to use a public time server, you should consider using a server pool. Server pools are public addresses from which you will get a random, pooled time server by requesting the time. This way you will not have to split your requests. There are public server pools available for many regions which you may use to avoid problems mentioned above.
See pool.ntp.org to find your nearest server pool. For example, if your server is located within Germany you can connect to 0.europe.pool.ntp.org.