Depends on what you want to optimize for.

These are different suggestions based on assumptions of your situation. Assuming you have access to fairly decent tap water (Europe and US) I'd follow Todd's recommendation for Zerowater as thats what I daily drive. I used to rely on Brita but it does not do much...just slightly improve taste and remove the worst heavy metals.

Running tests on my water, there are still traces of some impurities like bacteria and some metals after Zerowater filtering.

I am also about to start testing the Aquatru system that he also recommends.

For Zerowater the filters are quite expensive (15$ per filter) and I have been peeved at how they get used up quickly (I have around 200PPM in my tap water and they last 75-90 days) and still dont filter everything out fully...but subjectively I can't get over how I like the taste and since im not on the west coast im in the more lucky group where Zerowater is good enough. Maybe Todd was also in this situation?

My family does not like it but I love it. It tastes like "flat" water and becomes filled with bubbles if let to sit out for a short time but its worth it because it "feels" so clean.

Sorry if this observation is unscientific but once you have physically removed impurities, you still will have some semblance of subjectivity. I try to target distilled water taste as a point of reference and Zerowater comes close.

I have worked on a system where I first filter out using a Brita filter and then run it through the Zerowater to help improve the results but the problem with this is that my water is not bad enough for the brita's simple activated charcoal to really help reduce so in my case it actually has ended up giving mixed results.

If you lived in the west coast where the water is typically around 400ppm then you'd extend the life of the Zerowater filter quite a bit by doing this trick but for me, well I have to try something else.

But at that point maybe the Aquatru is better which is why I am trying it. For me, the Aquatru is just for curious comparison as this whole journey is reaching nutjob levels for me at this point and im not elon musk levels of rich(these water tests are not cheap)...the Zerowater is good enough for my usage because I live in a suburb away from any industrial places/poorly managed municipality(no major pathogens, just correcting the taste/eliminating any traces of metals and dirt).

In reality, Brita is probably fine but I want that taste of flat water now that I have gotten a craving for it. Every time I drink something else like Brita or bottled water it just tastes weird.

In reality others have told me if one is to spend the dough on Aquatru, you might as well get a under sink reverse osmosis system installed. Takes up less room and is cheaper. I got a good deal on a open box unit so I decided to go that route. Sorry I dont have results yet.

If you are concerned about pathogens that could make you sick, then it becomes tricky. I have traveled to Pakistan and lived in places there where the tap water makes you sick. I have relied on Grayl and based on my testing water and sending it out it seems to filter pathogens but unfortunately it is a massive pain to use. Do not rely on Zerowater/Aquatru for this as it will not help you. I am still in the search for an excellent under sink solution to eliminating pathogens + giving me the taste that I get with Zerowater. My ideal combination would be to have some sort of automatic Grayl + filtered afterwards with Zerowater. Beautiful tasting water but quite expensive. Might as well rely on water bottles at that point. Hope this helps a bit.