If you can get away with a single speed, and carrying weight is a big factor, nothing beats the <20lb pound Bike Friday pakit. The ride quality is amazing, as well, in spite of 16" tires. Depending on how much $ you want to throw at it, you can get that bike down close to 15 lbs.
If you get a brompton, consider the two speed. Very little weight penalty over the single speed and you may end up appreciating the extra gear if you use it to travel at all. Any of the bromptons with more than 2 speeds are pretty heavy to carry up stairs.