You can now customize the font for AdSense units, but there aren't many options: Verdana (the default font), Arial and Times (New Roman).
I think Verdana is still the best choice, irrespective of the font you use for a site. The text is more legible and it looks better.
Google's default font setting is sometimes Arial and sometimes Verdana. Here's an example from a 468x60 ad:
and an example from a 300x250 ad:
I agree with Ionut on the veranda choice, it looks much better than the others, at least at that small font size.
Also "non-serif" sans serif?
Ewww, yuk, I hate Tahoma! How abouts we get some Helvetica :)
Schmelvetica – haven't we all had enough of that in the last millennium? Let's have it in Hobo Outline Itals or Matisse Shadow Inverted, we might as well. C'mon you typo guys, get into gear, let's see some now-ness in the suggestions. I'd be happy enough with a reworking of Eric Gill's timeless masterpiece, in all its varietal glory, why not.
By the way, if you're near Morecambe call in at the newly renovated Midland Hotel at the end of the prom. It has some superb Eric Gill original work in its re-decorated murals and a ceiling painting to remind you of Michelangelo. His warts-and-all letterforms on the mural map (you can get a close-up) are delightful. Go on!
Me, I'm going back to bed.