Which "Text Style" will help keyword ranking?

[iProgram]

Hi,
Listed Text Style below. Which will help keyword ranking?
<b>keyword</b>
<strong>keyword</strong>
<u>keyword</u>
<i>keyword</i>

TIA.

[Brett_Tabke]

Use one from all four and there is a 100% chance you are right.

[iProgram]

I alway asked myself, will the following code dilute the "<b>" or "<font size="3"> tag:

<b><font size="3">keyword</font></b> VS. <font size="3"><b>keyword</b></font>

And how about
<p><font size="3"><b>keyword</b></font></p>