Kent Float Valve
I use 2, 1 in the sump, 1 on RODI outlet. 5 or more years now, and never once over filled my tank. I have an electric solenoid on my RODI tank for if my sump overfills i use as a backup. The electric shutoff has failed at least twice now when i'm cleaning the tank and it doesn't shut off the solenoid.
In the end, Crappy float valves have been the most reliable.