You will want to start with an API query like:
https://api.twitch.tv/kraken/streams/?game=Overwatch&limit=25&client_id=xxxxxxxxxxxxxxxxxxxxxxxx
This will have the API search for games matching Overwatch, and limit the results to the first 25. The order is by viewer count. Therefore, limiting to 25 you will get the top 25 most viewed streams (matching your search)in descending order.
doing a foreach loop through the 25 results we can store values from each stream. I grab the display_name (user’s set case of name) the name (lowercase version of the name) and a preview panel to be used (i used medium for the example)
I have removed my client_id and made the code viewable: http://isso.pro/twitchtest/overwatch.php.txt