Teledyne Laars makes a good unit. We have several of the Mighty Therm boilers and they are very simple and very reliable. Also quite easy to service and clean. http://www.laars.com/
I also have a couple of boilers made by Quikwater that have been very reliable. These are certainly more expensive, using a micro PLC and Honeywell temp control, but they work great in recirculation and high volume applications.