4.5:1 ratio is the minimum.
3:1 ratio is acceptable if it is "large" text
On mobile, a ratio of 7:1 (or 4.5:1 for large text) would be hitting it out of the park.
Listed are a couple of color contrast tools, one that is browser-based, and the other as a plug-in for Sketch:
Generate a compliant color palette for a style guide by selecting:
Background or Foreground Color
Font Weight & Size
Allows for review of color contrast directly within a Sketch project. Upcoming feature provides suggested colors that would meet conformance. (Like Colorsafe.co, above)