NavList:

I've used most of the modern brands over the years, all at sea on a merchant ship.  My favorite is a traditional C. Plath or C&P.  They are as accurate as you can get, and have excellent build quality and features such as polarized shades to fine tune the intensity of the celestial body under observation.

Tamaya would be my last choice of brands in metal sextants.  They are well built and nice to use, but cost the most and lack the extra features of the C&P.

Astra IIIB sextants are decent and probably the best value for the money.  They are a bit less accurate, but usually within the observer's standard deviation.

The Freiberger and Russian equivelants are decent as well, but I don't like their ergonomics and you can't mount other telescopes on them.

The one I truly find almost useless is the C. Plath Navistar professional from the late 70's and later.  A terrible sextant.  The shades were never the correct density for sunlines, and the ergonomics are the worst I've ever used.  A far cry from the traditional Navistar.

Jeremy