The question isn’t so much “which typeface” but which font will work – even if you use a typeface, the person at the other end might not have it, as such, it’s a good idea to use google webfonts, so you can either use the typeface locally, or the browser will D/L the font and use it from google – http://www.google.com/webfonts#
This limits your choice a little, but not a lot.
I’d say there is nothing wrong with Arial in terms of screen readability, but it’s a slightly old and tired font, which has been used lots and lots, and as such has certain connotations about it – so something like verdana, calibri, liberations sans, open sans or Cabin will work well.
The other thing is size, and context – having one typeface across the whole site tends to look a little dated, and less easy to navigate, it is normal to have a body type, and heading type, and an emphasis or quote typeface – then keeping this consistent across the whole site, without exception, tends to help.