I have the hydor mini heater, and it's one of my favorites.
It works pretty well and it's pretty constant too, no fluctuations that are harmful, it only changes by 2-3 degrees but that's from day to night.
The other two I have is a Tetra one, i've heard good and bad about it, mine is working well so far, hopefully it continues to be that way.
The other is Top Fin submersible heater, it works well too, the only thing is that I can't read what temperature it is at on the switch, but I use a thermometer and check it if I decide to change the temp on the heater. It stays in the lower/mid 80s.