you cannot put "<" inside XML attributes, that doesn't work even in normal HTML. You'll have to use entities and then turn off escaping in <f:selectItem>:
<f:selectItem itemEscaped="false" itemLabel="ng.l<SUP>-1</SUP>" />
unfortunately this doesn't work for the native select - its appearance is based on the OS and it doesn't support HTML and <rich:select> will always escape the labels. On the other hand it works for checkboxes and radioboxes.