what is the most effective and cost efficient way to heat basement rooms via electricity?
Efficiency

Cheap to install, expensive to run: 

All electric resistance heaters are basically 100% efficient, no more, no less.  *Mind you, that's at converting electricity to heat.  Back at the power plant, things are not so efficient**, and that's it's much cheaper to just use gas, rather than have them turn it into electricity first.*  An electric heater spends all its energy as heat, and since they don't have or need vents, it all winds up in the room.

Expensive to install, cheap to run: 

If you want to exceed 100% efficiency by a wide margin, use a heat pump.  This actually does work.  It is taking heat from outside and concentrating it.  However this is a lot like an A/C system (in reverse) and so it's hard to have fine zone-control.   If you want zone control, there's a product called a "mini-split heat pump" with no ducting and a little unit in each room. There needs to be freon piping back to an outside evaporator.   

Thermostats

On the unit:  Most heaters have a kit that lets you put a thermostat right on the heater.  The heater's heat causes the thermostat to misread, so not a great idea.   You also have to turn off a bunch of thermostats when you go home.  

Line-power thermostat: They are cheap and can drive several heaters, so zone control is easy.  It switches the heavy electric power cables going to the unit.  Those big wires make some heat inside the thermostat which again can cause it to misread, which is why they're infamous for being sloppy.  It also requires the heavy power cables make a big detour to go to the location of the thermostat.  This adds length, cost and energy loss while in use. 

Common 24V thermostats: Electric heaters can also be used with common 24V thermostats, including "smart" thermostats like the Nest or Ecobee. This involves running the heavy cables in a beeline to the heater, and putting a common 24V relay either in the heater or down at the service panel. The installation is more complex but the most flexible.  You can even integrate the electric heat with a gas or heat pump system, using it as a stage of auxiliary heat. 

A heat pump absolutely requires a 24V thermostat, and a fairly sophisticated one at that!

Quality

Resistance heaters are very simple things.  It's possible for them to be both cheap and high quality.  Cadet heaters are notoriously cheap, but I can't honestly say I've ever seen one fail, including some that were horribly abused by bad placement. They just keep working. 



** A thermal power plant (that burns gas or coal to spin turbines) is 37-40% efficient; 60+% of the heat value is lost in the machinery.  So is nuclear, but nobody cares about thermal efficiency of nukes.  Hydro is near 90% efficient at turning falling water into electricity.  Wind and solar use "found energy" so efficiency is not critical. Electric heat is a good "dump" for wind/solar systems that have topped up their batteries.