When creating a website, it is important to create a sitemap so that a search engine can find, crawl and index all of your website's content. a sitemap makes site creation much more efficient and simple. A sitemap also helps site developers (assuming you have any) understand the layout of your website, so that they can design according to your needs.
A wireframe is another important step in the web design process. Creating a website is like building a house. To build the house, you first need a foundation on which to build it upon. Without that foundation, the house will collapse. The same goes for a website. If you create a wireframe as a rough draft of your website before going through and adding final touches, the entire design process will become much easier. If you do not first create a wireframe, the design process will be considerably more difficult, and you are more likely to encounter problems later on.
To me, the wireframe is the most important due to the fact that is necessary in order to create a good website. In order to create a sitemap, you first need a rough outline of your website. Without that outline, creating a sitemap is impossible.