Best brands by reputation (deservedly so) are Ebo Jager and Tronic (some also swear by VisiTherm). With all due respect to others who may have had good luck with them, Rena Cal heaters are usually not included on lists of most reliable heaters.
No heater is perfect or immune from failure, but if you want to play the odds, you're safest with Ebos and Tronics. Do a search for 'heater' on this board and you'll find lots of discussion of different brands.
HTH,
Jim