1. Saikou: IMO this is the best anime streaming app for Android. It supports multiple sources, subtitles, picture-in-picture and so much more. It is open source. You can also read manga on it!
2. Aniyomi: Aniyomi is an unofficial fork of Tachiyomi, a dedicated manga reader. So you can stream anime and read manga from a single app. Like Saikou, Aniyomi supports multiple sources.
3. Cloudstream: Cloudstreaming is a great streaming app which you can not only for streaming anime but also for watching thousands of TV shows and movies. The app also has features similar to the above mentioned apps but does not have a manga reader.

If you use any other anime streaming apps, please mention it by leaving a comment.