What is the best way to convert seconds into (Hour:Minutes:Seconds:Milliseconds) time?

For .Net <= 4.0 Use the TimeSpan class.

TimeSpan t = TimeSpan.FromSeconds( secs );

string answer = string.Format("{0:D2}h:{1:D2}m:{2:D2}s:{3:D3}ms", 
                t.Hours, 
                t.Minutes, 
                t.Seconds, 
                t.Milliseconds);

(As noted by Inder Kumar Rathore) For .NET > 4.0 you can use

TimeSpan time = TimeSpan.FromSeconds(seconds);

//here backslash is must to tell that colon is
//not the part of format, it just a character that we want in output
string str = time .ToString(@"hh\:mm\:ss\:fff");

(From Nick Molyneux) Ensure that seconds is less than TimeSpan.MaxValue.TotalSeconds to avoid an exception.

For .NET > 4.0 you can use

TimeSpan time = TimeSpan.FromSeconds(seconds);

//here backslash is must to tell that colon is
//not the part of format, it just a character that we want in output
string str = time .ToString(@"hh\:mm\:ss\:fff");

or if you want date time format then you can also do this

TimeSpan time = TimeSpan.FromSeconds(seconds);
DateTime dateTime = DateTime.Today.Add(time);
string displayTime = dateTime.ToString("hh:mm:tt");

For more you can check Custom TimeSpan Format Strings

If you know you have a number of seconds, you can create a TimeSpan value by calling TimeSpan.FromSeconds:

 TimeSpan ts = TimeSpan.FromSeconds(80);

You can then obtain the number of days, hours, minutes, or seconds. Or use one of the ToString overloads to output it in whatever manner you like.