Hands down the Hydor Theo 25 watt Heater. It's actually not that expensive considering it's fully submersible, has a light indicator, temperature adjustable, changeable positions, shatterproof glass, etc; it's honestly like any of those fancy heaters just shrunk down to 25w. It's also got a neat slender design that most appreciate.
Gonna "me too" the Hydor Theo 25w. It's very reasonably priced for all the features. Especially the automatic shut-off feature for if the water level gets too low. I may or may not have forgotten to unplug it before doing water changes a couple times...
Though neither it or the Aqueon Pro on my 10 gallon have accurate thermostats. Based on reading reviews, it's a pretty common problem.
Hydor's decent, but it doesn't have a very accurate thermostat IMO. I've had at least five of those heaters that malfunctioned - after turning down that thing to 69 F, I ended up with a temperature of 84 F. Not a good ending.
If you're not tight on money, I'd go with the Neotherm Heater. It's plenty accurate with the thermostat, 3-year warranty, and has a built-in thermometer (which, is also accurate - confirmed by my three other thermometers since I'm paranoid).
EDIT : Sorry, didn't realize how old this thread was! If your hydor malfunctions, might want to get the Neotherm