With me having 5 tanks in my house i can confidently say that ive tested alot of diffrent heaters.
i recently had two 300w heaters(cheap chinese imports)in my oscar tank these were terrable as the temp setting very unstable jumping between 22deg C & 28deg C and almost impossible to keep a constant temp.
I went out to the LFS and bought a "Jager 3609" its a eheim product rated for a 600L to 1000L tank its fairly large(22inches),its got a calibration button on top of heater so you can keep it calibrated to perfection.
Ive replaced my two 300w chinese imports with this one heater and i can honestly say its the best heater ive ever used,i reccomend this product to anyone who requires a new heater.
This product is a bit exspensive,i paid $100 for it here,but its well worth the money as my oscars health is more important than a 100 bucks.