Italics. Probably because I'm old, I see bold generally as bad form. I have transgressed very occasionally myself though, and some of the posters make a good point about italics being lost when quoted in a later post. Must dig out some of the old style guides as a reference (well, maybe if I can be bothered).
Underlining is just about unforgivable - it was originally intended as a direction to the printer to put the phrase in bold. Bold and underlining, is, therefore, a typographical tautology, and dreadfully bad form.
Actually, any time that someone applies more than one of underlining, italics, capitals and bold to the same word, to me it shows that they simply can't get their message to be taken seriously enough just on the worth of the message, and are resorting in desperation for some other method of trying to convince you that their opinion really, really matters, and is really, really right.