A HEPA filter is doing the heavy lifting within any air purifier, it is therefore the most important ingredient. Activated Carbon is well known for removing smells/cents, but it does more than just that: It also neutralizes VOCs and some other gaseous pollutants which can travel through a HEPA filter.
This is why I currently have a commercial air filtering solution instead of my box fan + 3M filter, it has suspended Activated Carbon around the HEPA filter fabric. I don't know how I'd achieve that with my box fan solution, and I've not seen anyone else try to address it.
Other air purification technologies have safety issues (e.g. ionizers, ozone) or just outright don't work (e.g. UV light inside a channel). HEPA and Activated Carbon have measurable upsides with no downsides aside from their cost. They're the best we have today in spaces occupied by people.