I would say the salinity and ATO Sensors... ( i have two ATOs on my set-up) APEX FMM during the Day and Icecap ATO at night, since the tank is in the same room that I sleep in...(Nothing worst than being woken up to a loud FMM.ATO at 3am)
This is just not APEX related... Controllers dont fit the bill,bad wording now as most devices offer up that... its more monitering what is going on.. then if you want Skynet ability you can live or die by that. I for one just want information provided to my with the tech we have.
I would say that depends on your situation. If you're keeping coral the definitely ALK. If not, I'm really not sure (I've always kept reefs vs FOWLER's).
The Trident was a godsend when I got it, checking the big 3 parameters multiple times a day with no input needed from me? Yes please!