We will have a reverse osmosis system in our house that will send water to the refrigerator and to a dedicated faucet at the sink. We picked a Delta beverage faucet.

I buy water by the gallon. The plastic waste and the lugging are just too much. I've been checking into under sink options and can pass on some basic info but not a recommendation.

First, you should have your water tested. Many filter systems offer special cartridges to remove specific substances. The range of filtration can be from simple taste improvement to near-purification. The hardness can't be mitigated by most sink based filters and that's commonly addressed by a whole house water softener. A reverse osmosis system will remove nearly everything, but at the expense of expending more waste water than they yield of purified water. Also, the more 'stuff' removed, the more it taxes the RO membrane. These are slow to filter and have a tank that sits under the sink.

I plan to buy a two cartridge system that will be under the sink with a dedicated tap installed above. These can be a DIY project but typically need a 1/2" hole drilled in the sink or counter. There are similar units that can sit on the counter to dispense. If the guest house is not heavily used, it's an easier installation option.

A quick search will bring up some of the well known brands. Some have conveniences like filter replacement indicators or quick release housing. Filter life and replacement costs as well as flow rate are factors to consider. Popular brands have the convenience of finding the replacement filters easily.

I'm tending toward Aquacera. I emailed them my water analysis and felt confident in their recommendation of the Nilus 2 for my water profile. They went on to say that RO wasn't necessary in my instance, nor was there a need for a three stage unit (they sell all). I appreciated the approach to not oversell. I had planned on something more involved and more costly. Another online company recommended the max in filtration to the tune of thousands of dollars and RO which is too wasteful for me. My well water is hard but not very hard, no odor, taste, color issues but had a safe but scant reading of nitrate. The Aquacera model above can be fitted with a nitrate filter in addition to their seemingly thorough Cerametix filter. From the link you can go to a lab report to read all that it filters. Of course chloramines and fluoride are not present in well water, but it appears to address most other potential contaminants.

Some online stores sell Aquacera units and replacement filters but they don't always seem to bundle specific filter options with the units. Here's a link to the manufacturer's countertop unit.

We have very hard, nasty water and have tried a couple systems. The only thing that’s worked so far is reverse osmosis at the kitchen sink only. It has a dedicated faucet. We rent the unit - not sure of the price because we also rent a charcoal carbon filter thingy for whole house (which is not good for drinking and cooking). But I think it was another $10 a month (approximately) to add the RO. The water tastes great. Faucet is cheap, bought online from Lowe’s but it does the job.

Thanks all, I will look into doing a test. The undersink option is what appeals to me at this point.

We have had one for years. And a hot water heater. I love it

Not sure how this model performs since we haven't installed it yet and I don't know if it's sufficient for well water, but this is the filter we chose for our new coffee bar. EWS Max Filter

We went with this model because we don't have a lot of room at our bar sink so I didn't want to install a separate water dispenser on an already crowded sink deck. The EWS came highly recommended because it has a good flow rate and can connect to any faucet.

We got it at a great price at a local plumbing store. For those in the Bay Area, we purchased ours at Rubenstein's for a fraction of the on-line price.