Key word here is POTENTIAL. Currently my personal favorite is Luna due to how cute and social she is. Not only that, but very open to trying a variety of new experiences. Plus the fact that I enjoy staying up late and enjoy the late would help a lot as well I am sure. I am sure other people would argue that Celestia or Book Horse is Best Waifu and that is fine.
Now as to Discord...he has what I would say Near Unlimited Waifu Potential.