Stardew is probably one of the most (if not the most) popular game ever made with MonoGame
AAA studios don't really use MonoGame.
Another one would be Bastion - code is even open source [0].
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[0]: https://www.supergiantgames.com/blog/bastions-open-source-br...
By sales it is the 17th most popular game accross all systems and all history. But I think Terraria beats it even on Mono
Second would be Terraria (originally made for XNA).
Their point wasn't about MonoGame. Do AAA studios really not use any open source tools?