The "BEST" are built by spectrapure, but you WILL pay for them. AquaticReefSystems puts together a great system, but again,not cheap. What you need, depends on the quality of your starting water, and how much water you need. Some people get by fine with just a DI system, others, just a good RO, while many of us need the added help of a RO/DI. Everyone I know who's bought the Aqua-Safe units, has ended up buying a better unit within a couple of years. I STILL have the Spectrapure 3 stage unit I bought 15 yrs ago. (think it's time to change cartridges or membrane yet?) It has been in a box for a couple of years, since I bought the ARS 100 gpd Oceanus.
All of the RO sellers just assemble components built by others. Spectrapure goes a bit farther and tests each membrane, and the ones that test over 95% rejection, are what they call "Select" and go into their higher end units.