Who makes the lightest seats for a Porsche 911?

The lightest seats? If you look across the range of race shells from Sparco, Recaro, Momo, Kirkley, etc… you'll find the top offerings from each will probably have differences so small as to be such that you could claim just about any one of them to be “the lightest.”

But like the question you asked in regards to wheels, the real question is not going to be which is the lightest (which will be just a few pounds total), but what is a workable seat that also happens to be light. Race shells are wonderful and offer fantastic support for the driver, and happen ot be very light… but they don't work well with standard belts, so you need to also install 5 or 6 point harnesses… and if you’ve a harness you should have a full role cage since a harness prevents sideways movement in the event of a rollover (and thus your head will always be in the way of a collapsing roof)… and so on. Shells also wear out faster than seats and will need to be repalced. And they offer tiny cushioning.

A basic sports seat from Sparco, Momo, or Recaro, will offer more support, and should trim a solid 20-30 pounds off of the stock offering (though you'll be trading power and heat and any other seat options that come from the factory).