My biased answer would be the widget that my company created in-house: the Context Menu a.k.a. the Right-Click Menu for usage on data grids. You just right-click on records, and you get a menu to run different microflows, and even sub menus.
It's my favorite because it replaces the default behavior for selection/microflow running on data grids, and it's a new UI piece that's different from everything else.
That being said, I could not go a day without the FormLoader. Maybe this widget is the most obvious choice but it deserves recognition because of its continual practicality and benefits.
I really like and use the Input Reference Selector v1.8.2 a lot. It would be nice if this one ended up in the application instead of an app.
From a consistent UI stand point I do always make a modification to this one because with the standard reference selector you have always the button to open the select form. I now just make a grid of two cells where the reference selector goes in the left cell and the standard reference selector goes in the right one. I then adjust the cells that the right one is real small so only the select form button is displayed. This way you get a consistent UI with the standard button.