There is no "best engine" for games, all engines offer different functions. Find the best engine by determining what you want to create for your game. Unity is great for 2D and 3D android games and offers plenty of features.
did you actually buy unity??? That's very expensive software. If your using the free version of unity you probably aren't going to be in the position to produce a game worth while on any device. Free unity is mostly for learning. You cant upload any models or really use 90% of the softwares functionality. If you want to actually produce games that are sold on app stores Unity requires you to buy a monthly license for $75.
Anyway, you can make games for both IOS and android using Unity. I personally have used both Unity and UE4 for school/hobbiest work. UE4 is free which is great however Unity is very easy for someone used to just typing code to learn. UE4 kinda requires you to learn another language called blueprints which is just rly visual coding.
TLDR; Unity is ****ing expensive but its great software. Used free version to produce maze based game with just a buncha shapes in my highschool independent study. I really wish I could find a way to get my hands on the paid version without using money. I personally love it and would highly recommend.