The guy asked for our favorite UI fonts, I told him. You guys are allowed to a have a different favorite than me, it's OK. Some of ya'll need a hug.

Pretty sure I made my choice clear. It has a ton of weights, it scales nicely, large xheight contributes to readability. It's a geometric sans-serif that would literally work in 90% of layouts. If San Francisco works, Circular works.

If you don't want to use the system font then I recommend Circular. It has many weights, scales wonderfully, and it has a tall x height which helps readability.