Universal TV Remote Control
Universal TV Remote Control from Codematics is one of the highest rated remote control apps on the Google Play Store. Mainly because they have an excellent easy to use remote, a clean interface, and the app works without a hassle. Not to mention it's not filled with 15 ads like some other remote apps.
This app works with LG, Google Chromecast, Roku devices, Android and Apple TV, and more. Just connect to the same WiFi, find the device, and go. That includes Google Cast built-in.
Then, if you have an IR Blaster it gets even better, as they have a huge database of supported devices. The best part though is there's no setup. Simply select your TV brand and start using the remote. It does all the work. And if it can't find your device, email the developer and they'll add it.