For body copy, I most commonly recommend Verdana. Arial is also a good, legible choice.
For headlines, you can stick with Verdana or Arial – but don’t mix and match these together. For example, don’t use body copy in Verdana and then headlines in Arial, because it will look like the text used for the headline is too “skinny”.
Or, if you’d like to add some variety, you can go with a serif font for the headlines and subheads. Two of the best serif fonts to read online are Georgia and Times.