Right now there are several different styles of head tubes (HT) that can go on a bike. The biggest determining factor in deciding on which HT is right for you is the type of fork you are going to get. In this post I am not going to get into the pluses and minuses of tapered or straight steerer tubes, rather that they exist and determine the type of HT you will need.
If your type of riding does not require the stiffness that tapered steerer tube offers then a fork with a straight steerer will be your ideal choice. With a ridged fork this is often (but not always) determined by whether the fork has disc brakes or not. For a suspension fork a majority of quality forks are tapered.
With a straight steerer you have four options: