Strongest? I'd look at titanium heaters. They can get expensive though, especially the good ones with controllers. I don't have any experience with the big expensive ones, my titanium heater was like $50 and works exactly how I want it to. I can't recall the brand name, and it's in the basement, but I got it at aquacave. It has fin in the name.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry