in terms of sheer value, i second the dyi kalk dripper made from my kids koolaid bottle, and my 150lbs of base rock i got cheap as heck which is now all live rock.
In any case, I can't pick one since there is always something else "better" out there I want to get next. That is why this is a hobby, it takes away your money and time.
I guess best value or best utility, may be something that made the most noticable difference. I think that the best thing I've purchased so far has been my VHO setup. That's what gave me the most noticable difference in my reef.
About the refractometer: I heard someone had a swing arm that registered 1.021 but a refractometer showed his water to be 1.30!!!
I think I'm going to toss my swing arm in the trash...
My float switch, which controlls my auto top off. You can get one for less than fifty bucks and it saves a lot of time. Next on my list would be my magfloat, simple and cheap.