Wisdom (Animal Handling), though I'd use Dexterity or Strength in place of Wisdom quite a lot, I think.
Depends on what your trying to achieve.
Trying to ride hard over a long distance? Well that is a constitution check.
Trying to calm spooked horses? Well that is wisdom.
Trying to reach down to the ground to grab a bag as you ride by? Well, that is dexterity.
Want to pathfind your way down a difficult slope, well that might be wisdom or intelligence.
Thanks for the info. Still new to the way skills work in 5e.
That's your issue, there is no ride skill in 5e. The Combat chapter has a section on mounted combat (page 198 PHB, 76-77 Basic) and the Adventuring chapter (page 182-183 PHB, 63-64 Basic) talks about mounted travel. The mounted combat section is the only description I've found of any game mechanics for riding. Basically, a lot of PF ride skill is handled by the mount having been "trained to accept a rider."
The animal handling 'skill' makes reference to getting your mount to do unusual things.