What is the mildest most efficient DE razor?

I sold mine to a buddy who now uses it daily: the Feather Popular. Mild, but really very efficient once you master the right angle and the polymer safety bars glide super smoothly as well. I could get a DFS effortlessly. One day I will get another, but until then the post-war Tech is my No. 1 daily driver.
I find that the Hone and Pils 101NE razors are mild. As to "efficient", both can be this, provided that you have adapted and honed (sorry!) your handling techniques to get the best out of them. If you don't, then you have a good way of scraping soap off your face, whilst perfectly preserving the bristles beneath.

Otherwise, I second the Krona, and a "Fat Handle" Gillette Tech with a Feather blade works well for me.
Interesting thread-

My mild but efficient razor would be a Weber (in my case the polished stainless example) with an Astra blade. Effortless shave you could do with your eyes shut but feel shaved for a day and a half!