I believe that Appium would be the best choice to go forward with. The primary reason being its large community as compared to other tools. Also, it has Selendroid bundled within it allowing it to have a large variety of features.

For some context on the community sizes, here are some testing tools' GitHub repos and stack overflow questions compared:
-> appium: GitHub stars = 12.1k, GitHub forks = 4.8k, SOverflow questions = 6.6K
-> Selendroid: GitHub stars = 813, GitHub forks = 451, SOverflow questions = 209
-> Robotium: no GitHub repo, SOverflow questions = 1k
-> Test project: GitHub stars = 11, GitHub forks = 3, SOverflow questions = not known

Now one may have a little confusion choosing b/w Appium and selendroid, but here's an excerpt from an ex-Google engineer - Twain Taylore's blog which clears the confusion up.

"In most cases, Appium is the right choice out of the two. It has a lot more versatility, wide- ranging support and comes with a Selendroid mode. Although Selendroid does a few things better than Appium and has a few similar features, Appium is comparatively a lot more versatile and has a larger community, making it a stronger choice for testing mobile apps. Plus, Appium bundles Selendroid, and falls back to it automatically when necessary. This means Appium is really the only tool you have to install. While it’s helpful to understand the differences between Appium and Selendroid, there’s no reason in most cases to install and run Selendroid as a standalone tool. "
(https://saucelabs.com/blog/selendroid-vs-appium-which-to-choose-for-your-mobile-app-testing)