I highly recommend Duo Security, the guys behind have a solid track record in the industry. They started it first with phones. Prior to that everyone else was using expensive RSA like secureid tokens that take more to deploy. Read the man page for ssh and you see Dug's name in there. Jon has a solid track record on the mobile side. They just developed Rekey to fix the Android master key vulnerability issue about 2 weeks ago.

We use Duo for various things around here. Love it. It's much more feature complete than Authy (which, unfortunately will not work for any of our use cases).

More importantly, the company is run by well know and respected security researchers (Dug Song and Jon Oberheide).

haven't used Duo Mobile but Authy has been great for us