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I use Giphy's imessage app a decent amount cause it has actual Gifs that are easily sendable vs a gif that plays as a video.

This is probably the most used imessage app TBH.

The third party keyboard SLASH is arguably a better overall option than how imessage apps are implemented. Very reminiscent of slack.
 
I use GIPHY and venmo all the time. The others intermittently but for me these beat the concept of swapping keyboards that came pre-iOS 10
 
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