i'm a big fan of the visi-therm heaters. i use a 50w on a 20g tank (of course, this is plenty for the climate i live in), and it is a picture of perfection. i also use a higher wattage model on a 50g tank. no problems at all. i've never used tronics, so i can't comment on them, though they do have a good reputation. i have used ebo jagers, and while i don't like them as much as the visi-therms, they are also dependable, tough heaters.
from my experience on this board and throughout the internet in general, i would say the consensus i've seen on heaters would be:
#1 ebo jager
#2 visi-therm
#3 tronic
then everything else