Best in wall speakers

The best made ones (in terms of build quality) are the Revels. Full die-cast metal enclosure makes them even stronger than the wall!

Being Revels, I suppose they sound pretty good too.

As per your requirement, I really think you should try psychohertz, speakers like galileo speakers and davinci speakers!

I enjoy Triad gold. Give them a listen. 

I’ve used B&W and Klipsch with succes. Those Triad Gold’s look promising, also Martin Logan’s Edge. Move up the lines based on your penchant for audiophilia!

In my experience most things that constitute stand-alones are relevant with in-walls. So say in choosing a more musical sounding receiver, integrated, or separates play a big part in doing 2-channel convincingly. Multi-channel usually is not as critical but can benefit. Also take in account if the speaker can benefit from a brighter feed or a sweeter sound like I found with the Klipsch and Vincent tubed hybrid.

Placement is critical for fronts with ear-level placement (even on sides of a display) for stereo imaging as you can’t toe-in for pinpointing. Using a brand/design that offers the same speaker for a Center is fine. For the surrounds placement from ear level to ceiling height should be taken into account. I’ve only used dipoles that sound quite convincing.

Of course if you’re able to audition with your choice of music make it priority! Have comparisons, as with in-walls a design can make the difference/benefit in sound not so much an improvement, went considering price and electronics used.