I'm with Hotblack on this one.

If I have to have only one prime lens, it's got to be 40mm (+/- 5mm). Can I have an f/1.4 please, with a multi-curved blade "circular" aperture stop for nice bokeh when stopped down?

But my favourite focal length for general photography is a lot harder to answer. I use focal lengths around 24mm and 85mm quite a lot. Both do things that are inconvenient or impossible with a "standard" lens. I'd feel deprived without either of these focal lengths, but not as deprived as I would be if I didn't have a "standard" focal length available.