My general approach is to get a room where acoustics are pleasant for conversation. That broadly means for me - like
@Senior NEET Engineer said - to make generous use of thick rugs, bookcases, plants, curtains, and furniture-stuff in general (and hanging non-reflective stuff on the walls. Paintings without glass work). The most important thing acoustics-wise is to avoid flutter echo in the high frequencies, which those things suppress reasonably well.
Beyond that it's really about having good speakers with good dispersion characteristics, and placing them well. If you can place the speakers at least 1 m away from sidewalls, and can listen in reasonable proximity (1 to 2.5 m from the speakers), it will sound good. If you have lots of flutter echo and you listen far away from the speakers, it will sound bad, particularly if the speakers have uneven dispersion (my personal experience is that one get away with bad dispersion or a bad room if one listens
very close to the speakers and places them away from room boundaries, as long as the direct sound on-axis is good).
Note that this is not uncontroversial. There are people who advocate that room treatments should work equally well on all frequencies, or who claim that side reflections should be suppressed etc. That is not my experience, though. I've listened in some rooms with lots of broadband absorption, and if often sounds dead and boring to me. Conversely I've also listened in bare and empty rooms with lots of flutter echo, and it also sounds bad to me. The rooms I like the most are somewhere in the middle between these two extremes - typically living rooms which are generously furnished and where there are lots of rugs and curtains.
My experience here is is broadly in line with several scientific studies (such as Bech 1994, and the more general summaries in Toole's book). People generally like reflections, it seems, as long as it's relatively timbrally correct. But there may also be genuine differences of preference here. One may learn to like one thing or the other.
In general we seem to be more sensitive to reflections and echos in some frequency ranges than in others, according to Bech 1994, and some other researchers. High-frequency flutter echo seems to be particularly bothersome. So I try to have a room where sound in a broad sense feels pleasant over time - be it conversations, music over loudspeakers or with live instruments. To get such a room may include using room treatment products, or it may not. Given that most loudspeakers have a more even horizontal than vertical dispersion, it often makes most sense to "treat" ceiling reflections for example. The problem is that building such room treatments takes time and money, and afterwards it is difficult to do A/B comparisons. It's good advice to visit different kind of listening rooms first, and get a sense of what one likes.
I've yet to listen in rooms with generous use of professional diffusion products. If I was to make use of room treatments myself, I would probably go the route of diffusion rather than absorption.
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