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Linear (of linear.app) has a highly polished UI. In fact they had a dedicated period for fixing just usability issues:

I wish they'd make more apps. I use Linear but with a team of 1, it's not exactly the best fit for me.

Linear has the best UI/UX of all the web apps I have used. After Gmail and Google Maps, I don't recall any other web app wowing me as much as Linear.