Oh my, that's a tough one! ATO is on top of list as well as Skimmer overflow protection. I think back over the past 30 years and I have always had a home made skimmer overflow protection as well as a home made ATO. But that would put both of these into the controller realm
. I must admit the controllers have made life easier since they integrate "everything" so I would have to say a controller. I have used Apex, RKE, Reef Angel, and waiting for my Archon. A fully integrated controller makes our life way easier. Todays controllers can control dosing, CaRx, float switches, web cams, web user interface, wave makers, ATO, Lighting, graphing/logging, heater, pH, ... you name it, with open source controllers, you can monitor and control.
Not going to get into which one is best as I have had a few and I liked them all
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My vote ..... Controller