I decided on Reverse Osmosis (RO) membrane filters, for MA water. These separate out particulate contaminants, as well as chemicals. Some chemicals, such as solvents, lead, chlorine, and god-knows-what, are bad, so I want them removed with extreme prejudice. However, some chemicals are good, such as fluoride and healthy minerals, which are also removed by the RO filter system. This is a known problem, so good RO filters include a "filter" stage which adds minerals back in. There are less effective filter systems (carbon and ceramic stack filters) and more effective systems (water distillers which condense steam). RO systems are the sweet-spot tradeoff in terms of maintenance vs purity. One strange thing about RO filters is that the RO membrane generates 5x-10x volume of waste water. You will hear this gurgling into your drain when the tank is filling. Oh, and they typically have a storage tank, so you can tap a gallon or two quickly, for drinking or cooking, and it will slowly refill the tank in an hour or so.
I settled on the very expensive but super-compact, Aquaphor DWM-101. I'm trying to make that link donate to the ASPCA charity. I'm not entirely sure my Amazon settings are correct though. Our constraints were that it had to be a good RO filter with pre-filters (for when they flush muddy goo into the supply without warning). Also, we need it to be compact to leave space for the sink disposal and a Franke HT-200 water heater. The RO has to have easily-replaceable filters, since the water quality is so terrible. Finally, this all had to fit into a single Franke LB-2000 hot-cold filtered-water spigot, since I didn't want to drill a second hole in the stone counter.
The filter works great!! Below are some before and after measurements. TDS stands for Total Dissolved Solids, a very general indicator of drinking water quality.
Distilled water should be 0-5 ppm.
Bottled spring water might range 5-120 ppm.
Bottled store water (e.g. Aquafina) is RO filtered, so it should be 10-50.
Carbon filtered should be 50-100 ppm.
Hard water is 100-200 ppm.
Average city water is roughly 200-400 ppm, which great variation above that.
EPA TDS max is 500 ppm.
Like most EPA limits, this is generous. Water with 500-600 ppm, such as Arizona, tastes disgusting according to blogs I've read. Our Cambridge water was TDS 350 ppm from the tap. It tastes like sucking on a radiator.
I researched water measurement tools. Here is a TDS-4 meter by HM Digital, for less than $25, which is pre-calibrated for temperature variation and drinking water, accurate to 2% in ppm. I recommend this. The reviews are quite educational. You can read multiple accounts where this tiny computerized stick is more accurate than $400-$2000 TDS meters, probably due to pro meters being more accurate but requiring constant calibration. You can take the tiny meter to restaurants. I do. Here is an EtekCity 2011 pH meter for less than $25, which I also recommend.
Unfiltered cold tap water, after running for a while: TDS = 350 ppm
Filtered with a Brita: TDS = 335 ppm
Filtered cold tap: TDS = 20 ppm
Filtered hot tap from our under-sink heater: TDS = 30 ppm
Standalone water heater filled with our filtered cold: TDS = 20 ppm
The filtered water has 6% = 20/350 of TDS, which means our filter removes 94% of impurities. Perhaps most importantly, my wife loves the taste of the filtered water. LOVES it! She drank four or more glasses a day and refused to drink anything else (other than a glass of milk for nutrition). We noticed that Washington water is very good, but at night, our apartment complex water gets metallic-tasting from standing in the pipes. We ordered another RO water filter and are installing it there soon.