I guess the right answer should be to not use any Fixed font size. If you are mostly concerned about readability and accessebility of your web site then don't use any fixed pixel font size. Period. Use percent. Normal font weight and size for information and main content, small or large for other contents. In this way the font size can be made bigger or smaller by the user, from the Browser option. If you use a fixed pixel value, and the user is using IE, then font size won't change even when the user wants to read the text in a bigger size. Though in Firefox, the font size will change according to the wish of the users, no matter if you use pixel or not. Using EM is a good option too for readability.
Saying that, I have to sadly confess that I do not go by the rules. I use fixed pixel size, because I like to have control over my layout. This works for most users, but it is definitely a bad practice.
. My favorite font is verdana and size 11px for most texts.
Georgia is a common font on most computers. It's little different looking too. You may try that too to see how it looks on your website.
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