According to this ochem website, HI is the most reactive and HF the LEAST. This is talking about HX addition to an alkene.
<a href="http://www.geocities.com/CapeCanaveral/Lab/4433/131A/ADDNRXN.HTM" target="_blank">http://www.geocities.com/CapeCanaveral/Lab/4433/131A/ADDNRXN.HTM</a>
<a href="http://www.geocities.com/CapeCanaveral/Lab/4433/131A/ADDNRXN.HTM" target="_blank">http://www.geocities.com/CapeCanaveral/Lab/4433/131A/ADDNRXN.HTM</a>