I think that a nice HTML sitemap is best to have people link to. XML is best to control what is indexed by search engines so it helps as well. Text is neither here nor there...
Only Yahoo recognizes text sitemaps 'officially'. HTML is supported by ALL search engines and humans can use it when visiting your website. It's just another webpage with links on it. XML was designed by the search engines, for the search engines. There are a few other formats with obscure uses such as OPML which can be imported into your blogroll (then it links to your pages from your blog), RSS which works in feed readers, and ROR which lost steam as XML became more popular.