In my experience, and also from talking to other experienced hobbyists I trust, the one heater that keeps coming up again and again as the most reliable you can buy in unbreakable, full submergible heaters is the VisiTherm Stealth line.
200W heater currently on sale at Dr. Foster Smith
for $17.79. Many LFS
will carry the Stealth line of heaters but my experience is that often the prices in pet stores is nearly DOUBLE the price DFS sells them for online. I needed to buy a 50W one for my 10 gal
tank recently; PetSmart was selling it for $29.95 while the online price from the link above was $15.29 -- half the cost. Even with shipping, it's probably cheaper to order it online, and if you need some other supplies as well and can put in a big order, then the shipping price goes down even more.
I currently have two tanks with these heaters and have had absolutely no problems.
After looking at that site again, I also ran across listings for something called the VisiTherm Stealth Deluxe heaters
. Hard to tell exactly what the difference between the two is, as I've never seen a "deluxe" model in person. Anyway, thought I'd mention it.