My parents have a Carmor down draft stove. It is the most impressive I've seen amongst many. It loads through a round top lid and has two tubes dropping down to the bottem of the box for the air feed to the coals area. It gets the absolute most heat out of the wood and longest burns that I've ever seen.It has great control on the draft which makes all the difference in the world.
A lot of questions to answer for a wood stove, but look at the Vermont Castings Defiant. Seems I saw where they have a very high efficiency rating. This is a guess, and it is the stove I have as the only heat in a 24x30 'almost all-glass' sun room. This stove has a catalytic converter in it so 'burns' and gets heat from the gases in the smoke.