Any and all of them, including the Canon L. With very low f/numbers the optical problems increase exponentially, especially when required to cover full frame. 4/3rds format makes things very much easier.
Even f/1.2 is pushing it, regardless of cost. The current Canon L 50 1.2 has its quirks and the 85 L 1.2 has such a huge light cone that it won't fit through the mirror box and those wonderful out of focus highlights get clipped! Which rather defeats the point IMHO.
I think f/1.4 is the practical limit and there are some really very good f/1.4 lenses about. Pushing for that extra half a stop or even a whole stop is pretty pointless when the depth of field is so shallow that barely 1% of the image is actually sharp anyway! Who needs it with today's high ISO?