It depends on why you want to underline a piece of text.
Is it a heading? Use <h1>-<h6>.
Is it an emphasized word? Use <em>.
Is it text that has been inserted after a document edit? Use <ins>.
Is it an instance of a term that is being defined? Use <dfn>.
And so forth. If HTML doesn't have an element type for the purpose then use a <span>.
Then you can alter the presentation of that element with CSS as described above.