I'm designing a UI for an app in Mac OS (Yosemite) which font is the default font? or which font is widely used in the latest Mac apps?
Yosemite uses Helvetica Neue.
The documentation says you should refer to it as the system font. Presumably this has the advantage that your application will survive any underlying OS change and continue to use a font which matches what the system uses.
The system font is a specially optimized version of Helvetica Neue that gives your text unmatched legibility, clarity, and consistency. Yosemite tunes Helvetica Neue to balance aesthetics and layout compatibility so that most apps can look great without making any changes. In particular, apps that use Auto Layout to express the relationships among views in the UI can respond automatically to slight changes in font metrics.
Always designate a system font by name, such as
boldSystemFontOfSize:. If you instead specify “Helvetica Neue” in code or in Interface Builder, you get the nonoptimized version of the font, which is suitable only for the content of a document. Using the nonoptimized font in your UI can have unexpected results.
The easiest way, and the one most mac users are going to appreciate, is sticking with the default font, which is San Francisco as of 10.11, Helvetica Neue for 10.10 and Lucida Grande for previous versions.
If you must use a different font, make sure to check the license: most paid fonts will charge quite a lot more for app licenses.