Any recommendations for the most efficient electric boiler?  (this will be for radiant heating throughout house)

Our plumber recommended Allied 61kBtu

but I've read good things about Slant Fin

open to any suggestions

All electric boilers run the same efficiency- you'll have to decide based on other factors.

Any suggestions welcome

try the Lasco ceramic Tower Heater
A baseboard heater spreads heat using a combination of conduction and convection. Conduction is the transfer of heat from the heating element to the air immediately around it (from a heated surface to a gas, in this case air), while convection happens when the heated air, which expands and becomes lighter as it is heated, rises out of the heater and draws colder air in from below. Air currents resulting from this convection draw the heat up into and around the room where the heaters are located.