The best filtration comes from Reverse Osmosis systems. RO systems reduce dissolved solids in the water such as nitrates, nitrites, barium, etc. The RO membrane is the part of the RO system that reduces dissolved solids. Regular filters cannot reduce dissolved solids. Here is a easy to use RO system:
PuROTwist 4000 Gold Series Reverse Osmosis System and PuROTwist 4000 Replacement Filters
You can use the Culligan US-600 and hook it up to your main faucet. However, if you have it connected to your main faucet, you will want to use the D-20 filter. If you use the CBR2-10 filter, the flow rate will be very slow because the CBR2-10 reduces more contaminants. For any high contaminant reduction filter, you will need a separate drinking water faucet because the filter requires more contact time thus slower flow rate.
Culligan US-600 and Culligan US600 under sink water filter system for drinking water
Another under sink option that isn't an RO system but comes with a separate drinking water faucet and a really good filter for contaminant reduction is the Culligan US-EZ-4:
Culligan US-EZ-4 UnderSink Drinking Water Filter and Culligan USEZ4 Under Sink Filter 01019054
Thanks,
Culligan water filters, replacement cartridges, and home water filter products