Quickest way to rate takes in Premiere?
First, you need to adjust the metadata display. In the panel menu of the project panel, select the option Metadata display. In the Metadata Display window, activate the Rating display under Basic. This way, you get your five-star rating system right in the project panel (you can change the position of the rating column using drag-and-drop). Unfortunately, I haven't found a way to assign shortcuts to this star rating system.

Another option, if you are dead set on primarily working with the keyboard, are label colors. You can change the available label colors in the program preferences. You could, for example, use a color code as your rating system (e.g. green → blue → yellow → orange → red). Then, you can assign shortcuts to these colors: Press CTRL+ALT+K to bring up the Keyboard Shortcuts window. The color shortcuts can be found under Edit → Label.