If you make no assumptions whatsover about what you want to photograph the 50mm (FF 35mm Camera) is the most flexible lens for several reasons.
1) Generally, it's hard to take a decent portrait with a wide angle
2) Generally, it's hard to take a decent landscape/citiscape with a telephoto
3) It's faster than virtually all other lenses so greater possibilities there.
4) The wider aperture can give you greater dof effects
Plus, it tends to be smaller, lighter, cheaper and better quality than zoom lenses.
So all in all it's got a lot going for it which was why it was the standard lens with which most SLR's were supplied for decades. (And long before people started talking about a 'kit' lens.)