Rena Smart Heater is the best one that I have used. It keeps the water in my cichlid tank within a degree, all the time. I also have a Via Aqua Titanium heater which works well in my 58 gallon tank sump, the outside temperature dial is kick ass too. I have a Jager heater that I basically got for free from drs foster and smith because they never sent me a return label to ship it back and its been 2 months. They both worked fine but the temp dial on the Jager is super hard to change and online it says they are fully submersible and they probably are but the box warns you not to submerge it past the water line level on the heater.
Out of those three, the Rena smart heater is my favorite for stability, but the dial is also hard to change. The Via Aqua is really really easy and the heater is fully submersible and it has a seperate temp sensor that you can mount away from the actual heater unit.