I personally use Aqueon Adjustable heaters. Ive read too many things about other heaters malfunctioning and never had problems with any of mine for my 4 tanks of all sizes. Reading the stories of heater malfunctions gave me a case of heater paranoia. So I'm stuck on one heater. Also super easy to get in case of an emergency since Petco carries them.
I have an Eheim heater for my 46 gallon and I love it.
For a 55 gallon id opt for the Eheim 200w