Wow these are odd responses. My Auto Top off is great as it maintains my salinity and makes life for me much easier but I wouldn't trade an RO/DI for it. Same with my skimmer, I'd take the bigger skimmer over ATO anyday. Halides also, wouldn't trade those for ATO.
Same goes for a doser or my RK2. I think if my corals could talk they would say:
yeah, but i doubt you would say your anus to be your favorite of all body parts jhale. unless you have issues like that or are into that sorta thing......
I always liked the floating dog with the diving helmet better than the guy in the clam.
Yeah but that's why I tried to break it down in terms of upgrades. It seems like most people get into reefing trying to cut corners. Most people may not start out with halides, or the skimmer that they end up keeping. Many on this board (myself included) didn't start with an RO/DI, and had flow provived by a few Maxi-jets. I tried to pose the question more along the idea of which upgrade or major purchase was the best for you.
Being that I was wise enough to use tap water for one year I'm biased that way. My Ro/DI and phosban reactor were the best things I did for myself and my reef. If I wasn't such a fool and started with RO/DI I would probably feel differently.
I agree with your statement though, if I had to pick my most valuable piece of equipment, well I couldn't. It WOULD be like saying "which organ is most important." However, I could say which educated addition/upgrade I made to my existing reef that had the biggest impact.
I think the post was stated the wrong way, should be the best upgrade you made to your tank, not what is the best thing you bought, because you might not know it till you upgrade to something better (or worse in some cases) =0)