I'm probably Odd Man Out in that I site font sytle "Bookman Old Style" "Times New Roman", "Serif" with default line heights, 150% for H1, 130% H2, 130% H3 (italic) a standard P and a "small" of P at 90% for the "fine print".

If Bookman is installed on the user's machine then site is large and in charge. Times also works.

I reserve Helvetica styled fonts (Arial/Verdana) for counterpoint to the site body. Main concern is line WIDTH, not HEIGHT, so div styled for that.

I come from a print background and just can't get into sans-serif (anything) as body text. Just my personal preference. I sometimes browser override sites which want to give me sans serif. And since I can do that I know that others may be doing exactly the same (opposite) to my sites. Ultimately it's a crap shoot... you do the best you can while always remembering that Content is King!

Serif fonts are better for readability.
Verdana (and other sufficiently spaced fonts) work well as a secondary choice.

I never use a smaller font size than the default, and typically employ a 1.5 line-height.

Verdana tends to look awfully large at the default size, which is why using a font like Tahoma or Trebuchet (with Verdana as a fall-back) is good.

'Times New Roman', serif;
'Trebuchet MS', Verdana, Arial, Helvetica, sans-serif;

Do not use Times as a fall-back font since:
1) Times New Roman is an alias for Times on all newer systems
2) There is an awful (and somewhat common) Windows font called Times, but without any relation to the Mac Times