I used a Top Fin Submersible Heater, 25W, in my 5.5 and in my 10, a 50W. I've seen a lot of people using this heater, and in fact just yesterday bought a 150W for my 29 gal. Very trustworthy, you can set the temp range then check with a thermometer, consistently keeps my tanks around 80. I've never seen anyone on here have a problem with them, and haven't had a problem with mine either.
EDIT: Found a link to what I use, if you can get to a petsmart
I think there have been good & bad experiences with just about any brand out there. I will usually go to Amazon & read the good & bad reviews of the brand/size I'm interested in, then price check. In any case I have, 1 Eheim Jager & 1 Aqueon (both 1500W in my 46g), 3 Top Fin heaters (2 in 10g tanks, 1 in a 5.5g) & 1 Petco heater in a 2.5g. I have not had one problem with the operation of any of the brands I have. The Top Fin heaters are the ones that came with the Top Fin 10g aquarium kit. One has been running non-stop for about a year.
I like Hydor THEOs, Visi-Therms, and Eheims.
I have 2 Hydors, which I LOVE. They don't malfunction and always go to the desired temperature. I got mine off eBay, I've heard some people have trouble from Amazon, though. All the heaters I listed are adjustable. Oh, and I recommend a 25-50 watt for your tank.
I like my two Aqueon Pro heaters - they are not made of glass and are of the type that allow you to set the temperature. They do cost a bit more than their glass equivalents, however.
That being said, the rest of the ones I have are of the glass variety and do fine, though I have noticed that the one on my desk at work makes popping sounds when it turns on.
The only one I have not used so far is the Fluval I picked up for my 6.6 gallon, and that is because, if I'm reading it correctly, it is NOT fully submersible like the others. The dial part must remain outside the tank. None of my tanks are tall enough for it except the 36, which of course requires a much larger one.
Have not been overly impressed by the little 5W nonadjustable ones that they market for smaller betta enclosures. They do keep the water a BIT warmer than ambient, but would barely push the water into the lower end of the betta range for me.
I just replaced my Top Fin 50w because it was giving me temps in the morning of anywhere from 74-84. I suppose it may just have been a dud. I got a Jager to replace it and I love it. It keeps my temp really stable, even overnight. If it isn't heating the water to the temp you set, then you can calibrate it to the correct temp instead of playing with the temp dial until you get it right. The only thing is it's a PITA to move the temp dial.