IMHO keyboard ergonomics (standard layout, don't wanna further complicate things) come down to 2 things:
1. Force required. You want low-force. This is how much physical work you put into actuating a switch. Less force, less strain = more ergonomic.
2. The bottom-out. You want a soft landing. This is easier on the nerves in your fingertips, which, trust me, hurt really really bad after long typing sessions.
Thus, in my mind, the most ergonomic standard keyboard would be something akin to a brown or red MX, PCB mounted, with a layer of foam or w/e to soften the last half millimetre of the keystroke.
This way, because it's mechanical and the activation point is high, you don't have to bottom out. If you do bottom out, it's dampened and won't hurt your fingertips.