I think everyone has a lens "ground zero". It may be they know it already, or more likely don't realise, but on investigation would find that they gravitate towards the same focal range often. For me it is about 60-70mm on a full frame. 

I have often said that I find the 40mm focal length to be the perfect all-rounder, but looking at the question as a "world coming to me", rather than "me going to the world" scenario, a slightly longer, than 50mm focal length feels perfect for what I like to do.

How do I know?

There are a couple of clear indicators. 

1) It is roughly the middle of my comfortable range (35/50/*/90/150)

2) I have used a couple in the past (Canon 40mm on a crop and Sigma 30mm on M43, 135mm on a Pentax 67) and they have felt the least "opinionated" of lenses to me.

3) As a portraitist, I like a little compression and tightness, probably more than the 50mm offers (one of my issues/challenges with the 50mm focal length is it's indecisive nature) , but as wide as I can get for versatility and naturalness. 

I honestly feel I could work as a full time portraitist with just this focal length.

The perfect foil for it is the 40mm, offering probably the second least difficult focal length.

A neat pairing is the excellent 23/40mm (40/65) pancake pair from Canon, mounted on a crop body. Very compact, versatile and high quality.