Star Wars Galaxy of Heroes is currently my favorite mobile game to play. I have been playing this game for the past year, which is pretty long for me to stick with a mobile game. The game is currently free on the app store and can be played without ever spending a single dollar if you choose.

The general idea of the game is to collect "heroes" from the Star Wars universe and use them in turn based battles. Collecting enough hero shards will allow you to unlock the hero. The game continues to open new modes to you as you level up your profile and keeps you hooked as you improve your heroes. There are multiple ways to improve your characters:

Collect additional hero shards to improve your heroes to 7 stars
Level up your heroes to max level of 85 which correlates to your player level.
Equip gear and mods to your heroes to improve their stats.

Here is just a sample of the heroes I have been using and you can find in this game.

The game has a number of modes to play. This keeps the game fresh and presents multiple opportunities for rewards.

You have your normal Cantina, Light Side, and Dark Side battles where you play against the game using the two different types of energy.
The Squad Arena allows you to play against other players and earn rewards based on your rank.
Galactic War allows you to play against the game where each match increases in difficulty and then resets the next day.
Not shown in the screenshot are ship battles. These unlock further into the game.

A big part of the game is joining a guild. Your guild will be able to take part in raid battles against a Rancor or an AAT. There are major rewards from working together with your guild like hard to find gear or special character shards.

Events are happening on a weekly basis in the game. Sometimes the event gives you access to a special hero, unique gear or mods, or in-game currency.

The game is constantly adding new modes and new characters as the Star Wars universe expands. I recommend giving it a try. Warning, you might get hooked!