The ‘official’ Google font and one that’s similar to well known Helvetica, Roboto is a good standard font, although there’s nothing particularly special that I like about it, it just does it’s job.
Sometimes you can get away with using a serif font on screen, and very rarely can you do that. But the free Google font Droid Serif does just this. If you must use a serif font on screen, this is the one.
If you have a spare $1,000 lying around to spend on one font family, then you can’t go past Circular by Lineto. Outrageously expensive, but all the more beautiful, Circular is gaining traction as a popular font with it’s use in the Air BnB logo.