what were the best applications that were built on twitter's API before twitter cut off the API access?

Thinkboi

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https://techcrunch.com/2009/02/19/the-top-20-twitter-applications/

i think TwitPic had the most monthly active users 
https://en.wikipedia.org/wiki/TwitPic

I think TweetDeck probably one of the best showcasing what could have been. Although I believe Twitter did acquire them before cutting the API access.