I don't think there is a best, or at least a clear cut best. An outside boiler is pretty basic mechanically with differing bells and whistles so the sales boys have something to coo about and impress the masses. You can read the occasional story on the Internet about how some poor innocent fellow had a Mfg stuck him with a unit that only burns wood and smokes but no heat comes out or it rotted out in 5 weeks and fell into a thousand pieces or some other BS but I suspect all of that is just that, BS or poor installation practices. Basic decisions you have to make are. One, do you want to go with a Mfg that has been in business for a while and thus can expect he will still be in business when you need parts in 10 years or save a buck and go with a upstart? Two, stainless steel vs carbon (don't expect a conclusive answer here) Third, natural draft and large water capacity vs forced draft and less water to heat (dont expect a conclusive answer here either) My choices were carbon steel, forced draft and Woodmaster