The question was not specific if the post is in the shop or in the field.
If your building a wooden fence, you might very well set all of the posts, string a line to mark the tops of the posts, and then cut the posts to level the top of the fence.
If that is the case , a speed square and circular saw is a good way to go. You notice I didn't say only way to go.
I prefer to finish the cut with a handsaw because I find it quicker than having to level the square and circular saw with the previous cut.
If you are in the shop, a band saw with a miter gauge is hard to beat.